Free ASCP MLS Exam Practice Questions Mock Test: Part 30 – Microbiology – Diseases of the Immune System

1. ELISA (Enzyme-Linked Immunosorbent Assay): Used as an initial screening test to detect HIV antibodies (and sometimes antigens like p24) in the blood. It is highly sensitive but may yield false positives.

2. Western blot: A confirmatory test used after a positive ELISA result to detect specific HIV proteins (antibodies). It has high specificity and ensures accuracy in diagnosis.

The other tests are not used for HIV diagnosis:

• b) Hemagglutination test: Used for detecting antibodies or antigens in viruses like influenza or measles, not HIV.

• c) Agglutination test: Used for various infections (e.g., typhoid, rheumatoid factor) but not standard for HIV.

• d) Coombs test: Detects antibodies against red blood cells (e.g., in hemolytic anemia or HDN), not HIV.

Hereditary deficiencies in early classical complement pathway components (C1, C4, and C2) are strongly associated with an increased risk of developing autoimmune diseases, particularly a systemic lupus erythematosus (SLE)-like syndrome.

The other options are incorrect:

• a) Pneumococcal septicemia: Associated with deficiencies in terminal complement components (C5-C9), which impair membrane attack complex (MAC) formation and lead to recurrent Neisseria (e.g., meningococcal) infections, not pneumococcal.

• b) Small bowel obstruction: Not linked to complement deficiencies; it is a surgical/mechanical issue.

• d) Gonococcemia: Associated with terminal complement deficiencies (C5-C9), which increase susceptibility to Neisseria infections (e.g., gonorrhea or meningococcal sepsis).

The wheal and flare reaction is a classic sign of immediate hypersensitivity (Type I). This response typically occurs within 15-30 minutes after allergen exposure in a skin test (e.g., for allergies to pollen, foods, or drugs).

The other options are incorrect:

• b) Delayed hypersensitivity (Type IV): Appears 48-72 hours after exposure (e.g., tuberculin skin test), characterized by induration and redness without wheal/flare.

• c) Anergy: The absence of an immune response (e.g., no reaction to skin tests due to immunosuppression).

• d) Arthus reaction: A localized Type III hypersensitivity (immune complex-mediated) that occurs 4-12 hours after exposure, featuring edema, necrosis, and hemorrhage—not wheal/flare.

Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive disorder characterized by the classic triad of:

1. Immunodeficiency (recurrent infections due to defective T and B cell function),

2. Thrombocytopenia (low platelet count with small platelets, leading to bleeding),

3. Eczema (inflammatory skin condition).

This syndrome is caused by mutations in the WAS gene, which encodes a protein involved in cytoskeletal organization and immune cell signaling.

The other options are incorrect:

• a) DiGeorge syndrome: Features thymic hypoplasia, cardiac defects, and hypocalcemia (not eczema or thrombocytopenia).

• b) Bruton agammaglobulinemia: Involves absent B cells and antibodies, leading to recurrent bacterial infections (no eczema or thrombocytopenia).

• c) Ataxia telangiectasia: Characterized by cerebellar ataxia, telangiectasias (dilated blood vessels), and immunodeficiency (not eczema or thrombocytopenia)

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of various autoantibodies. Antinuclear antibodies (ANA) are a hallmark of SLE, with over 95% of patients testing positive for high titers of ANA. These antibodies target components of the cell nucleus (e.g., DNA, histones, ribonucleoproteins) and are a key diagnostic criterion.

The other options are incorrect:

• b) Decreased serum immunoglobulin levels: SLE patients typically have elevated immunoglobulins due to hyperactive B cells and autoantibody production (e.g., high IgG). Hypogammaglobulinemia is not a feature.

• c) High titers of anti-smooth muscle antibodies: These are associated with autoimmune hepatitis, not SLE.

• d) High titers of anti-mitochondrial antibody: This is a marker for primary biliary cholangitis, not SLE.

DiGeorge syndrome is caused by a deletion on chromosome 22 (22q11.2 deletion syndrome). This results in defective development of the third and fourth pharyngeal pouches during embryogenesis, leading to:

• Absence or hypoplasia (underdevelopment) of the thymus.

• Consequently, severe T cell deficiency (due to lack of T cell maturation in the thymus).

• Associated features include congenital heart defects, hypocalcemia (due to parathyroid gland hypoplasia), and characteristic facial abnormalities.

The other options describe defects in other immunodeficiencies:

• a) Lack of B cell maturation: Characteristic of Bruton’s agammaglobulinemia (X-linked).

• c) Defective NADPH oxidase in neutrophils: Seen in Chronic Granulomatous Disease (CGD).

• d) Overproduction of IgM: A hallmark of Hyper-IgM syndrome (e.g., due to CD40L mutation).

Antihistamines such as Benadryl® (diphenhydramine) work by competitively blocking H1 histamine receptors on target cells (e.g., in blood vessels, smooth muscles, and nerves). This prevents histamine (released during IgE-mediated allergic reactions) from binding to these receptors and triggering symptoms like itching, swelling, vasodilation, and bronchoconstriction.

The other options are incorrect:

• a) Depress IgE production: Antihistamines do not affect IgE synthesis; they target downstream effects of histamine.

• b) Block antigen binding to surface IgE: Antihistamines do not interfere with allergen-IgE interactions on mast cells/basophils.

• c) Bind histamine: Antihistamines do not directly bind or neutralize histamine; instead, they block the receptors histamine acts on.

Myasthenia gravis is an autoimmune disorder where the body produces antibodies that attack and damage or destroy the receptors for acetylcholine (a neurotransmitter) at the neuromuscular junction. This disrupts communication between nerve cells and muscles, leading to muscle weakness and fatigue. While the autoimmune response is systemic, the primary and most affected organs are the skeletal muscles, causing symptoms like drooping eyelids, double vision, difficulty swallowing, and generalized weakness.

The other options are incorrect:

• a) Heart: While the heart is a muscle, it is not the primary target; cardiac involvement is rare in myasthenia gravis.

• c) Kidney: The kidneys are not typically affected in this disorder.

• d) Lungs: Respiratory muscles can weaken (a life-threatening complication called myasthenic crisis), but the lungs themselves are not the primary target.

Anti-thyroid peroxidase antibodies (anti-TPO) are autoantibodies that target thyroid peroxidase, an enzyme involved in thyroid hormone production. High titers of anti-TPO are most commonly associated with autoimmune thyroid diseases, particularly

The other options are incorrect:

• b) Systemic lupus erythematosus (SLE): Linked to antibodies like ANA, anti-dsDNA, or anti-Smith, not anti-TPO.

• c) Chronic hepatitis: Associated with anti-smooth muscle antibodies (autoimmune hepatitis) or viral markers, not anti-TPO

Bruton’s agammaglobulinemia (X-linked agammaglobulinemia) is a primary immunodeficiency disorder caused by a mutation in the BTK gene, which is essential for B-cell development. This results in a near-total absence of B cells and significantly reduced levels of all immunoglobulin classes (IgG, IgA, IgM, etc.). Without antibodies, patients suffer from recurrent bacterial infections (e.g., sinopulmonary infections, otitis media) starting in early childhood. They are also vulnerable to certain viral and enteroviral infections.

The other options are incorrect:

• b) SCID (Severe Combined Immunodeficiency): Affects both T and B cells, leading to severe infections from all pathogens (bacterial, viral, fungal), not just bacterial.

• c) DiGeorge syndrome: Primarily involves T-cell deficiency due to thymic hypoplasia, but B-cell function may be partially impaired; bacterial infections are not the sole hallmark.

• d) HIV: A secondary immunodeficiency that gradually destroys CD4+ T cells, impairing both cellular and humoral immunity over time; it is not characterized solely by absent B cells or only bacterial infections.

• Primary immunodeficiencies are genetic disorders present at birth that result in defects in the immune system. SCID is a classic example, often caused by genetic mutations that impair the development and function of T cells and B cells.

• AIDS (Acquired Immunodeficiency Syndrome) is a secondary immunodeficiency caused by the HIV virus, which attacks the immune system after birth.

• Systemic Lupus Erythematosus (SLE) and Rheumatoid Arthritis are autoimmune disorders, not primary immunodeficiencies. They involve overactive immune responses rather than congenital defects in immune function.

Anti-double-stranded DNA (anti-dsDNA) antibodies are highly specific and diagnostic for systemic lupus erythematosus (SLE). They are a key serological marker used in both diagnosis and monitoring of disease activity, as their levels often correlate with flares, particularly in lupus nephritis (kidney involvement).

The other options are incorrect:

• a) Rheumatoid arthritis: Associated with antibodies like rheumatoid factor (RF) and anti-CCP, not anti-dsDNA.

• b) Multiple sclerosis: Diagnosed through clinical presentation, MRI findings, and cerebrospinal fluid analysis (e.g., oligoclonal bands), not anti-dsDNA.

• d) Sjögren’s syndrome: Linked to antibodies like anti-SSA (Ro) and anti-SSB (La), not anti-dsDNA.

Type 1 Diabetes mellitus is an autoimmune disorder where the body’s immune system mistakenly attacks and destroys the insulin-producing beta cells in the pancreas. This leads to a lack of insulin, requiring lifelong insulin therapy for management.

The other options are incorrect:

• a) Diabetes insipidus: Results from a deficiency of antidiuretic hormone (ADH) or kidney resistance to ADH, not related to the immune system or pancreatic beta cells.

• c) Addison’s disease: An autoimmune disorder targeting the adrenal glands, not the pancreas.

• d) Hashimoto’s thyroiditis: An autoimmune disorder attacking the thyroid gland, not the pancreas.

Hyper-IgE syndrome (also known as Job syndrome) is characterized by extremely high levels of immunoglobulin E (IgE) and recurrent “cold” skin abscesses (that lack warmth, redness, and pain due to poor inflammatory response). Other classic features include recurrent pneumonia leading to pneumatoceles (air-filled cysts in the lung tissue), chronic eczema, and coarse facial features.

• a) Wiskott-Aldrich syndrome: Involves low IgM, small platelets, and eczema, not specifically very high IgE.

• c) DiGeorge syndrome: Involves thymic hypoplasia leading to T-cell deficiency, cardiac abnormalities, and hypocalcemia, not high IgE.

• d) Bruton’s agammaglobulinemia: Involves a near-total absence of all immunoglobulins (including IgE) due to a block in B-cell development.

Graft-versus-host disease (GVHD) occurs when immunocompetent T cells from the donor graft (e.g., in bone marrow or organ transplants) recognize the recipient’s tissues as foreign and mount an immune attack. This requires:

1. The graft contains functional immune cells (e.g., T cells).

2. The recipient is immunocompromised (unable to reject the graft).

3. The recipient’s tissues express antigens not found in the donor (e.g., HLA mismatch).

Why not the others?

• a) The recipient’s immune system attacking the donor graft: This is graft rejection (host-versus-graft), not GVHD.

• b) Pre-formed antibodies in the recipient: These cause hyperacute rejection (e.g., via ABO or HLA antibodies).

• c) Immunosuppressive therapy: This is used to prevent GVHD or rejection but does not initiate it.

Hereditary angioedema (HAE) is an autosomal dominant disorder caused by a deficiency or dysfunction of C1 esterase inhibitor (C1-INH), a regulatory protein in the complement system. This leads to:

• Uncontrolled activation of the complement pathway (particularly C2 and C4),

• Excessive production of bradykinin (a vasodilator),

• Recurrent episodes of non-itchy, non-pitting edema in the skin, respiratory tract (potentially life-threatening), and gastrointestinal tract.

The other options are incorrect:

• a) Decreased activity of C3: Not characteristic of HAE; C3 deficiency is linked to recurrent pyogenic infections.

• c) Increased activity of C1 esterase inhibitor: The opposite is true; HAE involves reduced activity or levels of C1-INH.

• d) Increased activity of C2: HAE involves excessive consumption of C2 due to C1-INH deficiency, not increased activity.

Multiple sclerosis (MS) is an autoimmune disorder where the immune system mistakenly attacks the myelin sheath—the protective covering of nerve fibers in the central nervous system (CNS: brain and spinal cord). This demyelination disrupts nerve signal transmission, leading to neurological symptoms such as vision problems, muscle weakness, balance issues, and cognitive changes.

The other options are incorrect:

• b) Myasthenia gravis: Targets acetylcholine receptors at neuromuscular junctions, causing muscle weakness, but does not involve CNS demyelination.

• c) Rheumatoid arthritis: Primarily affects joints (synovial inflammation), not the CNS.

• d) Pernicious anemia: Caused by autoantibodies against intrinsic factor or gastric parietal cells, leading to vitamin B12 deficiency and peripheral neuropathy, but not primarily CNS demyelination.

Common Variable Immunodeficiency (CVID) is a primary immunodeficiency characterized by low immunoglobulin levels (leading to recurrent bacterial infections), but it is also strongly associated with autoimmune manifestations.

Why not the others?

• a) AIDS only: AIDS (caused by HIV) is an acquired immunodeficiency and increases susceptibility to infections/cancers, but it is not primarily linked to autoimmunity.

• c) Type I hypersensitivity alone: This refers to allergies (e.g., asthma, anaphylaxis), which involve immune overreaction to external antigens but not typically autoimmunity or combined immunodeficiency.

• d) Graves’ disease: This is a purely organ-specific autoimmune disorder (targeting the thyroid) and does not involve underlying immunodeficiency.

AIDS (Acquired Immunodeficiency Syndrome) is a secondary immunodeficiency because it is not inherited and is caused by an external factor—specifically, infection with the human immunodeficiency virus (HIV). HIV attacks and depletes CD4+ T cells, leading to a severe defect in cell-mediated immunity and increased susceptibility to opportunistic infections and cancers.

Why not the others?

• a) X-linked agammaglobulinemia: A primary immunodeficiency caused by a genetic mutation in the BTK gene, leading to absent B cells and antibodies.

• b) DiGeorge syndrome: A primary immunodeficiency caused by a chromosomal deletion (22q11.2), resulting in thymic hypoplasia and T-cell deficiency.

• d) Severe Combined Immunodeficiency (SCID): A group of primary immunodeficiencies (e.g., due to mutations in IL2RG, ADA, or RAG genes) causing defects in both T and B cells.

B-cells are responsible for producing antibodies (immunoglobulins), which are crucial for the humoral immune response against extracellular pathogens, particularly bacteria. A deficiency in B-cells leads to reduced antibody production, making the patient more susceptible to bacterial infections.

• Option a (decreased phagocytosis) is incorrect because phagocytosis is primarily carried out by cells of the innate immune system (e.g., neutrophils, macrophages) and is not directly dependent on B-cells.

• Options c and d (decreased or increased complement levels) are incorrect because complement proteins are produced mainly by the liver and are part of the innate immune system; their levels are not directly regulated by B-cells. However, antibodies can activate the complement system via the classical pathway, so complement activation might be impaired, but complement levels themselves are not typically decreased in B-cell deficiencies.

Systemic lupus erythematosus (SLE) is a systemic (non-organ-specific) autoimmune disease. It involves autoantibodies that target widespread cellular components (like nuclear antigens), leading to damage in multiple organs and systems, including the skin, joints, kidneys, heart, and blood vessels.

The other options are organ-specific autoimmune diseases:

• a) Type I diabetes mellitus: Targets insulin-producing beta cells in the pancreas.

• b) Hashimoto’s thyroiditis: Targets the thyroid gland.

• d) Addison’s disease: Targets the adrenal cortex.

Serum sickness is a classic example of a Type III hypersensitivity reaction. It occurs when foreign antigens (e.g., from antivenom, therapeutic antibodies, or drugs) trigger the production of antibodies (IgG or IgM) that form immune complexes with these antigens. These complexes deposit in tissues like blood vessels, joints, and kidneys, activating the complement system and causing inflammation, fever, rash, and joint pain.

The other types are incorrect:

• a) Type I: Immediate, IgE-mediated reactions (e.g., anaphylaxis, allergies).

• b) Type II: Antibody-mediated cytotoxicity (e.g., hemolytic disease of the newborn).

• d) Type IV: T-cell-mediated delayed hypersensitivity (e.g., contact dermatitis, tuberculin test).

Anti-CCP (anti-cyclic citrullinated peptide) antibodies are highly specific and diagnostic markers for rheumatoid arthritis (RA). These autoantibodies target citrullinated proteins in the joints and are present early in the disease, often before symptoms appear. They are used to confirm RA diagnosis and predict disease severity.

The other options are incorrect:

• a) Multiple sclerosis: Diagnosed through clinical criteria, MRI findings, and oligoclonal bands in CSF, not anti-CCP.

• c) SLE (Systemic Lupus Erythematosus): Associated with antibodies like anti-dsDNA, anti-Smith, and antinuclear antibodies (ANA), not anti-CCP.

• d) Graves’ disease: Linked to TSH receptor antibodies (TRAb), not anti-CCP.

The symptoms described—respiratory distress, vomiting, and hives occurring rapidly after a penicillin injection—are classic signs of anaphylaxis, a severe and immediate allergic reaction. This is a Type I hypersensitivity reaction, which is primarily mediated by IgE antibodies.

The other options are incorrect:

• a) IgG: Involved in Type II (e.g., hemolytic anemia) and Type III (e.g., serum sickness) hypersensitivity, not immediate anaphylaxis.

• b) IgA: Primarily found in mucosal secretions; not involved in anaphylactic reactions.

• c) IgM: Participates in primary immune responses (e.g., ABO blood group incompatibility) but not Type I hypersensitivity.

Graves’ disease is an autoimmune disorder where autoantibodies (thyroid-stimulating immunoglobulins, TSIs) bind to and activate the thyroid-stimulating hormone (TSH) receptor on the thyroid gland. This mimics TSH, leading to uncontrolled thyroid hormone production and hyperthyroidism. Symptoms include weight loss, heat intolerance, palpitations, and exophthalmos (bulging eyes).

Why not the others?

• a) Hashimoto’s thyroiditis: Involves antibodies against thyroperoxidase/thyroglobulin, destroying thyroid tissue and causing hypothyroidism.

• c) Addison’s disease: Autoimmune destruction of the adrenal cortex (e.g., anti-21-hydroxylase antibodies), leading to cortisol/aldosterone deficiency.

• d) Type I Diabetes Mellitus: Antibodies target pancreatic beta cells (e.g., anti-GAD, anti-insulin), causing insulin deficiency and hyperglycemia.

Waldenström macroglobulinemia is a type of B-cell lymphoma characterized by the malignant proliferation of lymphoplasmacytic cells in the bone marrow.

The other options are incorrect:

• a) IgG: Elevated in multiple myeloma (not Waldenström).

• b) IgA: May be elevated in some myeloma cases or IgA nephropathy, but not typical for Waldenström.

• d) IgD: Rarely elevated and associated with a minority of myeloma cases (not Waldenström).

Type II hypersensitivity reactions are antibody-mediated and involve IgG or IgM antibodies binding to antigens on cell surfaces or extracellular matrices. The primary mechanisms of tissue destruction include:

1. Complement activation: Formation of the membrane attack complex (MAC) causes direct lysis of target cells.

2. Antibody-dependent cellular cytotoxicity (ADCC): Effector cells (e.g., NK cells, macrophages) bind to antibodies via Fc receptors and destroy the target cells.

3. Opsonization and phagocytosis: Antibodies mark cells for destruction by phagocytes.

Why not the others?

• a) Release of histamine from basophils: This is characteristic of Type I hypersensitivity (IgE-mediated allergies).

• b) Formation of immune complexes: This defines Type III hypersensitivity (e.g., serum sickness, lupus nephritis).

• d) Activation of macrophages by T-cells: This occurs in Type IV hypersensitivity (T-cell-mediated delayed reactions).

The “window period” in hepatitis B infection is a specific time during convalescence when:

• HBsAg (surface antigen) has become undetectable (cleared from the blood).

• Anti-HBs (antibody to HBsAg) has not yet appeared or is at very low levels.

• The primary serological marker present is IgM anti-HBc (antibody to the core antigen), which indicates a recent acute infection.

This period is called the “window” because HBsAg (the common screening test) is negative, but the infection is still confirmed by detecting IgM anti-HBc.

Why not the others?

• a) Presence of HBsAg and HBeAg: This occurs in the early acute phase, not the window period.

• b) Presence of anti-HBs and anti-HBe: This characterizes resolution or immunity (after the window period).

• d) Absence of all serological markers: This suggests no infection or very early infection (pre-seroconversion).

Common Variable Immunodeficiency (CVID) is a primary immunodeficiency disorder. Its hallmark is a significant defect in the body’s ability to produce antibodies (immunoglobulins), specifically low levels of IgG, often accompanied by low IgA and/or IgM. This leaves patients highly susceptible to recurrent infections, particularly in the respiratory and gastrointestinal tracts.

• a) is incorrect because this describes AIDS (Acquired Immunodeficiency Syndrome).

• c) is incorrect as this describes an autoimmune condition like Hashimoto’s thyroiditis, not an immunodeficiency.

• d) is incorrect as hypersensitivity reactions are an overactive immune response, while CVID is an underactive one.

The VDRL (Venereal Disease Research Laboratory) test is a non-treponemal screening test for syphilis that detects antibodies against cardiolipin, a phospholipid released during tissue damage in syphilis. However, these antibodies can also be produced in other conditions, leading to a biological false-positive (BFP) result.

Why not the others?

• a) Gonorrhea: A bacterial STD that does not typically cause BFP reactions.

• b) Tertiary syphilis: Would yield a true-positive VDRL result (though treponemal tests are more specific).

• d) HIV/AIDS: May cause false-positive VDRL results but is less common than SLE. Acute infections (e.g., viruses) can cause transient BFPs, but SLE is the hallmark association for chronic BFPs.

Anti-glomerular basement membrane (anti-GBM) antibodies are autoantibodies that specifically target the non-collagenous domain of type IV collagen in the glomerular basement membrane of the kidneys and the alveolar basement membrane of the lungs. These antibodies are the hallmark of Goodpasture syndrome, a rare autoimmune disorder characterized by rapidly progressive glomerulonephritis (kidney inflammation) and pulmonary hemorrhage (lung bleeding).

The other options are incorrect:

• a) Systemic lupus erythematosus (SLE): Associated with antibodies like ANA, anti-dsDNA, and anti-Smith, not anti-GBM.

• b) Celiac disease: Linked to anti-tissue transglutaminase (anti-tTG) and anti-endomysial antibodies, not anti-GBM.

• c) Chronic active hepatitis (autoimmune hepatitis): Associated with anti-smooth muscle antibodies (ASMA) or anti-liver/kidney microsomal antibodies, not anti-GBM.

Peanut allergy is a type I hypersensitivity reaction, which is mediated by IgE antibodies. To confirm this allergy, the following in vitro tests are used:

1. Total serum IgE: May be elevated in allergic individuals but is non-specific.

2. Allergen-specific IgE: Measures IgE antibodies that are specifically directed against peanut antigens. This is a key diagnostic test (e.g., via ImmunoCAP or similar assays) to identify sensitization to peanuts.

Why not the others?

• a) IgG and allergen-specific IgG: IgG is associated with type II/III hypersensitivity (e.g., autoimmune disorders, immune complex diseases) or memory responses, not acute allergies.

• b) IgA and allergen-specific IgA: IgA is involved in mucosal immunity (e.g., in gut, respiratory tract) but not in type I hypersensitivity.

• d) IgM and allergen-specific IgM: IgM is produced in primary immune responses (e.g., infections) and is not involved in typical IgE-mediated allergies.

Type I diabetes mellitus is an organ-specific autoimmune disease. The immune system specifically targets and destroys the insulin-producing beta cells in the pancreas.

The other options are systemic autoimmune diseases, meaning they can affect multiple organs and tissues throughout the body:

• a) Systemic lupus erythematosus (SLE): Can affect the skin, joints, kidneys, brain, and other organs.

• c) Rheumatoid arthritis: Primarily targets joints but is systemic and can affect the lungs, heart, eyes, and other organs.

• d) Sjögren’s syndrome: Primarily affects exocrine glands (like salivary and lacrimal glands) but is systemic and can also impact the joints, lungs, kidneys, and nervous system.

• Hairy cell leukemia (HCL) is a rare B-cell malignancy characterized by:

  • Neoplastic B lymphocytes with “hairy” cytoplasmic projections

  • Monocyte-like features in morphology and some functional properties (e.g., adherence)

• Immunophenotype: CD19+, CD20+, CD11c+, CD25+, CD103+

• Clinical features:

  • Splenomegaly

  • Pancytopenia

  • Bone marrow fibrosis

Other options:

• a) Mast cells and B lymphocytes → not characteristic of HCL

• b) B and T lymphocytes → HCL is primarily B-cell in origin

• c) Granulocytes and monocytes → HCL does not derive from granulocytes

This is a well-established theory known as molecular mimicry. It occurs when antibodies or T-cells produced to fight an infection (e.g., a virus or bacteria) mistakenly recognize and react with the body’s own tissues because those tissues share similar molecular structures (epitopes) with the infectious pathogen.

• Why the other options are incorrect:

  • a) enhanced regulatory T-cell function: Regulatory T-cells (T-regs) suppress the immune response and prevent autoimmunity. Enhanced function would therefore protect against autoimmunity, not cause it.

  • b) diminished helper T-cell activity: Helper T-cell activity is crucial for activating most immune responses, including autoimmune ones. Diminished activity would more likely lead to immunodeficiency, not autoimmunity.

  • d) deficient B-cell activation: Since B-cells are responsible for producing antibodies (including autoantibodies), deficient activation would hinder the development of autoimmune diseases, not promote it.

Hashimoto’s thyroiditis is an autoimmune disorder where the immune system attacks and gradually destroys the thyroid gland. This damage impairs the thyroid’s ability to produce thyroid hormones (T3 and T4), leading to an underactive thyroid, or hypothyroidism. Symptoms often include fatigue, weight gain, cold intolerance, and depression.

The other options are incorrect:

• b) Hyperthyroidism: Refers to an overactive thyroid (e.g., caused by Graves’ disease, not Hashimoto’s).

• c) Addison’s disease: An autoimmune disorder targeting the adrenal glands, causing cortisol deficiency.

• d) Cushing’s syndrome: Results from excessive cortisol levels (e.g., due to steroid use or tumors), not thyroid dysfunction.

Bronchial asthma is primarily driven by immediate hypersensitivity (Type I), which is mediated by IgE antibodies. Upon exposure to allergens (e.g., pollen, dust mites, pet dander), sensitized individuals produce IgE that binds to mast cells and basophils in the airways.

The other options are incorrect:

• b) Antibody-mediated cytotoxicity (Type II): Targets specific cells (e.g., in hemolytic anemia), not typical in asthma.

• c) Immune complex (Type III): Involves deposition of antigen-antibody complexes (e.g., in serum sickness), not a primary mechanism in asthma.

• d) Delayed hypersensitivity (Type IV): T-cell-mediated response occurring hours to days later (e.g., contact dermatitis), not the rapid reaction seen in asthma attacks.

Rheumatoid arthritis (RA) is a chronic autoimmune disorder that primarily affects the joints. It causes inflammation, pain, swelling, and eventual damage to the synovial lining of joints, often leading to cartilage and bone destruction. Commonly affected joints include those in the hands, wrists, and knees.

The other options are incorrect:

• a) Kidneys: Not primarily affected by RA (though kidney issues can arise from medications used to treat RA).

• b) Lungs: While RA can have extra-articular manifestations (e.g., interstitial lung disease), it is not the primary target.

• d) Skin: Skin involvement (e.g., rheumatoid nodules) can occur but is secondary to joint disease.

The tuberculin skin test (Mantoux test) used to detect tuberculosis infection is a classic example of a Type IV hypersensitivity reaction. This type is also known as delayed-type hypersensitivity because the reaction typically peaks 48-72 hours after exposure to the antigen (tuberculin purified protein derivative, or PPD). It is mediated by T-cells (specifically Th1 cells and macrophages) rather than antibodies, leading to localized inflammation, induration, and redness at the injection site.

The other types are incorrect:

• a) Type I: Immediate hypersensitivity mediated by IgE antibodies (e.g., allergies, anaphylaxis).

• b) Type II: Antibody-mediated (IgG/IgM) cytotoxicity (e.g., hemolytic disease of the newborn).

• c) Type III: Immune complex-mediated (e.g., serum sickness, lupus nephritis).

Type III hypersensitivity is characterized by the formation of immune complexes (antigen-antibody complexes) that circulate and deposit in tissues, such as blood vessels, kidneys, and joints. This deposition triggers complement activation and inflammation, leading to tissue damage. Classic examples include:

• Systemic lupus erythematosus (SLE)

• Serum sickness

• Post-streptococcal glomerulonephritis

Contrast with other types:

• a) Type I: Mediated by IgE antibodies (e.g., allergies, anaphylaxis).

• b) Type II: Involves IgG/IgM antibodies directly targeting cell-surface antigens (e.g., hemolytic disease of the newborn, Graves’ disease).

• d) Type IV: T-cell-mediated delayed hypersensitivity (e.g., contact dermatitis, tuberculin reaction).

The combination of monoclonal spike of IgG (on serum protein electrophoresis), Bence Jones proteinuria (free monoclonal light chains in urine), and bone pain (due to osteolytic lesions) is classic for multiple myeloma.

The other options are incorrect:

• a) Burkitt lymphoma: A B-cell lymphoma associated with EBV, characterized by rapid growth and often involving the jaw or abdomen; it does not produce monoclonal IgG or Bence Jones proteins.

• b) Bruton disease (X-linked agammaglobulinemia): A primary immunodeficiency with absent immunoglobulins (not overproduction) and recurrent infections.

• c) Severe combined immunodeficiency disease (SCID): Features severe defects in T and B cells, leading to infections but not monoclonal proteins or bone pain.

Autoimmune diseases occur when the immune system loses its ability to distinguish between self and non-self antigens, a state known as a breakdown in self-tolerance. This leads to the production of autoantibodies and/or self-reactive T cells that attack the body’s own tissues.

• a) is incorrect: Autoimmune diseases are not contagious or transmitted between people. While infections can sometimes trigger autoimmune reactions in genetically predisposed individuals, the disease itself is not spread from person to person.

• b) is incorrect: Autoimmune diseases can be organ-specific (e.g., Type 1 diabetes, Hashimoto’s thyroiditis) or systemic (e.g., SLE, rheumatoid arthritis).

• d) is incorrect: Most autoimmune diseases are chronic conditions with no known cure. Treatment focuses on managing symptoms and suppressing the immune system to control the disease, often requiring long-term or lifelong therapy.

Goodpasture syndrome is an autoimmune disorder classified as a type II hypersensitivity reaction. It involves:

• Antibody-mediated cytotoxicity: Autoantibodies (IgG) specifically target the non-collagenous domain of type IV collagen in the basement membranes of the lungs and kidneys.

• This leads to complement activation, inflammation, and destruction of these tissues, causing rapidly progressive glomerulonephritis and pulmonary hemorrhage.

Why not the others?

• a) Immediate hypersensitivity (Type I): Involves IgE and mast cells (e.g., allergies), not autoantibodies against basement membranes.

• c) Immune complex deposition (Type III): Characterized by antigen-antibody complexes depositing in tissues (e.g., SLE, serum sickness), not direct antibody binding to tissue antigens.

• d) Delayed hypersensitivity (Type IV): T-cell-mediated (e.g., contact dermatitis, tuberculin reaction), not antibody-driven.

Ataxia telangiectasia is a rare autosomal recessive disorder characterized by:

1. Combined immunodeficiency (defects in T and B cells, leading to recurrent sinopulmonary infections),

2. Progressive cerebellar ataxia (loss of muscle coordination, beginning in early childhood),

3. Telangiectasias (dilated blood vessels appearing in the eyes and skin),

4. Increased sensitivity to ionizing radiation and higher risk of malignancies.

This syndrome is caused by mutations in the ATM gene, which is involved in DNA repair and cell cycle control.

The other options are incorrect:

• a) DiGeorge syndrome: Features thymic hypoplasia, cardiac defects, and hypocalcemia but does not involve ataxia.

• b) Bruton agammaglobulinemia: A pure B-cell deficiency with recurrent bacterial infections (no T-cell defects or ataxia).

• d) Wiskott-Aldrich syndrome: Involves eczema, thrombocytopenia, and immunodeficiency but not ataxia.

Chronic Granulomatous Disease (CGD) is a primary immunodeficiency caused by defects in the NADPH oxidase complex in phagocytic cells (like neutrophils and macrophages). This defect results in a deficiency in the oxidative burst, which is a crucial mechanism for killing certain bacteria and fungi. Consequently, patients suffer from recurrent, severe infections including skin abscesses, lymphadenitis, and deep-tissue infections (e.g., liver abscesses, pneumonia). A hallmark is the formation of granulomas (inflammatory nodules) as the immune system tries to wall off the persistent microbes.

• a) This describes defects in humoral immunity, like X-linked Agammaglobulinemia.

• c) This is a feature of Hyper-IgE Syndrome (Job’s syndrome).

• d) This is characteristic of an organ-specific autoimmune disease like Hashimoto’s thyroiditis.

Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive primary immunodeficiency disorder caused by mutations in the WAS gene. This gene encodes a protein essential for cytoskeletal organization and immune cell function. The classic triad of features includes:

The other options are incorrect:

• b) Arthritis, fever, anemia: Associated with autoimmune or inflammatory disorders (e.g., systemic juvenile idiopathic arthritis), not WAS.

• c) Rash, nephritis, arthritis: Suggest systemic lupus erythematosus (SLE) or vasculitis.

• d) Myopathy, neuropathy, rash: May indicate dermatomyositis or other neuromuscular disorders.

Sjögren’s syndrome is an autoimmune disorder that primarily targets the exocrine glands, such as the salivary and lacrimal glands. This results in symptoms like dry mouth (xerostomia) and dry eyes (keratoconjunctivitis sicca) due to reduced secretion of saliva and tears. It can occur alone (primary) or alongside other autoimmune diseases (e.g., rheumatoid arthritis or SLE).

The other options are incorrect:

• b) SLE (Systemic Lupus Erythematosus): A systemic autoimmune disease that can affect multiple organs (e.g., skin, joints, kidneys) but is not primarily characterized by dry mouth/eyes.

• c) Addison’s disease: Involves autoimmune destruction of the adrenal glands, leading to cortisol deficiency, not exocrine gland dysfunction.

• d) Rheumatic fever: An inflammatory reaction to Group A streptococcal infection, affecting the heart, joints, and other tissues, but not exocrine glands.

Type IV hypersensitivity, also known as delayed-type hypersensitivity, is mediated by T-cells (specifically CD4+ Th1 cells and CD8+ cytotoxic T-cells) rather than antibodies. It typically occurs 48–72 hours after exposure to an antigen. Contact dermatitis (e.g., reactions to poison ivy, nickel, or cosmetics) is a classic example, where haptens bind to skin proteins and trigger a T-cell-mediated inflammatory response.

The other options are incorrect:

• a) Serum sickness: A Type III hypersensitivity reaction (immune complex-mediated).

• c) Hemolytic anemia: A Type II hypersensitivity reaction (antibody-mediated cytotoxicity).

• d) Asthma: Typically a Type I hypersensitivity reaction (IgE-mediated immediate hypersensitivity).

The QuantiFERON-TB Gold test is an interferon-gamma release assay (IGRA) used to detect latent tuberculosis infection. It works by measuring IFN-γ released by sensitized T cells (specifically CD4+ T helper cells) when they are exposed to M. tuberculosis-specific antigens (ESAT-6, CFP-10, and TB7.7). These antigens are not present in the BCG vaccine or most non-tuberculous mycobacteria, making the test highly specific.

Why not the others?

• a) B cells: Produce antibodies but do not secrete IFN-γ in this context.

• b) Macrophages: Phagocytose pathogens and can secrete cytokines, but IFN-γ in this test is specifically from T cells.

• d) Neutrophils: Involved in innate immunity and do not produce IFN-γ in response to TB antigens.

Selective IgA deficiency is the most common primary immunodeficiency, characterized by markedly reduced or absent serum IgA (<7 mg/dL) with normal levels of other immunoglobulins. Key features include:

• Increased incidence of autoimmune diseases (e.g., celiac disease, rheumatoid arthritis, SLE) due to immune dysregulation.

• Recurrent sinopulmonary and gastrointestinal infections (as IgA protects mucosal surfaces).

• Increased allergic manifestations (not decreased, so option a is false).

• Normal secretory component production (but lack IgA to bind it, so option b is false).

• Prevalence is about 1 in 500 (not 1 in 50, so option d is false).

Hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis, is primarily caused by IgG antibodies. This condition occurs when an Rh-negative mother produces IgG antibodies against Rh-positive red blood cells from her fetus during a previous pregnancy. Unlike IgM antibodies, IgG antibodies are small enough to cross the placenta and enter the fetal bloodstream, where they attack and destroy the fetal red blood cells, leading to anemia and other complications.

The other antibodies are not primarily involved:

• a) IgA: Found in mucosal areas and not involved in HDN.

• b) IgE: Involved in allergic reactions (Type I hypersensitivity).

• d) IgM: Too large to cross the placenta efficiently; it is typically involved in initial immune responses (e.g., ABO incompatibility, which can cause a milder form of HDN but is less severe than Rh incompatibility mediated by IgG).

Selective IgA deficiency is the most common humoral (antibody) immunodeficiency, with an estimated prevalence of 1 in 500 people in the general population. It is characterized by significantly reduced or absent levels of serum IgA (and secretory IgA) while other immunoglobulin levels (IgG, IgM) are typically normal. Many individuals are asymptomatic, but some experience recurrent sinopulmonary infections, gastrointestinal issues, allergies, and an increased risk of autoimmune disorders.

The other options are less common:

• a) Bruton agammaglobulinemia: A rare X-linked disorder (∼1 in 200,000 males) with absent B cells and all immunoglobulin classes.

• b) IgG deficiency: Often occurs with other deficiencies (e.g., common variable immunodeficiency) but isolated IgG deficiency is less common than IgA deficiency.

• d) Wiskott-Aldrich syndrome: A rare X-linked disorder (∼1 in 100,000 males) involving combined immunodeficiency (T/B cell defects), not purely humoral.

The immunophenotype described—positive for CD2, negative for surface immunoglobulin (sIg), and lacking C3 receptors—is characteristic of T-cell disorders:

• CD2 is a classic pan-T cell marker (found on all T cells and NK cells).

• Surface immunoglobulin (sIg) is a B-cell marker (absent on T cells).

• C3 receptors (e.g., CR1, CR2) are primarily expressed on B cells and phagocytes (e.g., monocytes), not on T cells.

Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase expressed in immature B and T lymphocytes during early development. It is a key marker for:

• Acute lymphocytic leukemia (ALL), where leukemic blasts are often arrested at an early stage of differentiation.

• Distinguishing ALL from mature lymphoid malignancies (e.g., CLL) or myeloid leukemias.

TdT is detected via flow cytometry or immunohistochemistry and is highly specific for lymphoblasts in ALL (both B-cell and T-cell lineages).

The other options are incorrect:

• b) Adenosine deaminase (ADA): Deficiency causes a form of SCID (severe combined immunodeficiency), not a marker for ALL.

• c) G6PD (glucose-6-phosphate dehydrogenase): Involved in red blood cell metabolism; deficiency causes hemolytic anemia, not linked to ALL.

• d) Purine nucleoside phosphorylase (PNP): Deficiency causes T-cell immunodeficiency, not a marker for leukemia.

Wiskott-Aldrich Syndrome (WAS) is an X-linked recessive disorder caused by mutations in the WAS gene, which encodes the Wiskott-Aldrich syndrome protein (WASP). This protein is critical for cytoskeletal organization in hematopoietic cells. The classic triad of symptoms includes:

1. Immunodeficiency (recurrent infections due to defective T and B cell function).

2. Eczema (chronic inflammatory skin condition).

3. Thrombocytopenia (low platelet count with abnormally small platelets), leading to bleeding tendencies.

Why not the others?

• a) T cell receptor mutation: Not specific to WAS; T cells are dysfunctional but due to WASP defects, not TCR mutations.

• c) B cell deficiency only: B cells are affected (impaired antibody responses), but the defect is broader, involving T cells and platelets.

• d) High IgE levels exclusively: High IgE is seen in Hyper-IgE Syndrome (Job syndrome), not WAS. Though some WAS patients may have elevated IgE, it is not exclusive or defining.

Systemic lupus erythematosus (SLE) is a classic systemic (non-organ specific) autoimmune disease. It can affect multiple organs and systems throughout the body, including the skin, joints, kidneys, heart, lungs, brain, and blood cells.

The other options are organ-specific autoimmune diseases:

• a) Graves’ disease: Primarily targets the thyroid gland.

• b) Hashimoto’s thyroiditis: Primarily targets the thyroid gland.

• d) Type I diabetes mellitus: Primarily targets the insulin-producing beta cells of the pancreas.

The complement cascade involves several key immune mechanisms, but apoptosis (programmed cell death) is not directly mediated by complement. Instead, complement contributes to:

• b) Opsonization: C3b coats pathogens to enhance phagocytosis.

• c) Inflammation: C3a and C5a (anaphylatoxins) recruit and activate immune cells.

• d) Cytolysis: The membrane attack complex (MAC, C5b-9) forms pores in pathogen membranes, causing lysis.

Pernicious anemia is an autoimmune condition where the immune system attacks the gastric parietal cells in the stomach lining. These cells produce intrinsic factor, which is essential for vitamin B12 absorption.

The other options are incorrect:

• a) Anti-mitochondrial antibodies: Associated with primary biliary cholangitis.

• b) Anti-smooth muscle antibodies: Linked to autoimmune hepatitis.

• c) Anti-DNA antibodies: Characteristic of systemic lupus erythematosus (SLE).

Anaphylaxis is a severe, life-threatening systemic allergic reaction and is the classic example of a Type I hypersensitivity reaction. It is mediated by IgE antibodies, which trigger the widespread release of inflammatory mediators like histamine from mast cells and basophils. This leads to symptoms such as difficulty breathing, swelling, low blood pressure, and potentially shock.

• A marked decrease in CD4 lymphocytes and a decreased CD4/CD8 ratio is a classic hallmark of HIV infection, which is a viral-induced immunodeficiency (AIDS). The virus specifically targets and depletes CD4+ T-helper cells.

• Let’s review why the other options are incorrect:

  • a) is diagnostic for bacterial septicemia: Bacterial septicemia typically causes changes in neutrophil count (e.g., neutrophilia), not a specific, marked loss of CD4 T-cells.

  • b) may be seen in most hereditary immunodeficiency disorders: Many hereditary immunodeficiencies (e.g., Chronic Granulomatous Disease, X-linked Agammaglobulinemia) affect other parts of the immune system (phagocytes or B-cells) and do not characteristically present with this specific T-cell profile.

  • d) is only seen in patients with advanced disseminated cancer: While some cancers and chemotherapy can affect lymphocyte counts, this finding is not exclusive to cancer. It is most famously and characteristically associated with HIV.

Type IV hypersensitivity is also known as delayed-type hypersensitivity (DTH) because the reaction typically takes 48 to 72 hours to develop after exposure to the antigen. This delay occurs because the response is mediated by T-cells (specifically CD4+ Th1 cells and CD8+ cytotoxic T-cells) and macrophages, rather than antibodies. The activation and recruitment of these cells to the site of antigen exposure require time, leading to the characteristic delayed onset.

The other types are not delayed:

• a) Type I: Immediate hypersensitivity (e.g., allergies, anaphylaxis), mediated by IgE antibodies within minutes.

• b) Type II: Antibody-mediated (IgG/IgM) cytotoxicity (e.g., hemolytic anemia), which can occur within hours.

• c) Type III: Immune complex-mediated (e.g., serum sickness), typically developing within hours to days.

During active stages of systemic lupus erythematosus (SLE), immune complexes form and deposit in tissues, triggering widespread inflammation and complement activation. This consumption leads to decreased levels of complement proteins in the serum, particularly C3 and C4. Measuring complement levels (C3/C4) is a common way to monitor disease activity, as low levels often correlate with flares (e.g., lupus nephritis).

The other options are incorrect:

• a) Anti-nuclear antibody (ANA): Titers are typically elevated in SLE and may persist even during remission, though they are not reliable for monitoring active disease.

• b) Immune complexes: These are increased in active SLE due to autoantibody production and formation of antigen-antibody complexes.

• d) Anti-DNA: Anti-dsDNA antibody titers often rise during disease activity (especially in renal involvement) and are used as a marker for flares.

Primary biliary cholangitis (PBC), formerly known as primary biliary cirrhosis, is an autoimmune liver disease characterized by the destruction of intrahepatic bile ducts. The hallmark serological marker for PBC is anti-mitochondrial antibodies (AMA), which target enzymes within mitochondria (particularly the E2 subunit of the pyruvate dehydrogenase complex). These antibodies are present in about 90-95% of PBC patients and are highly specific for the disease.

The other options are incorrect:

• b) Anti-smooth muscle antibodies (ASMA): Associated with autoimmune hepatitis, not PBC.

• c) Anti-DNA antibodies: Linked to systemic lupus erythematosus (SLE), not PBC.

• d) Anti-parietal cell antibodies: Associated with pernicious anemia and autoimmune gastritis, not PBC.

Rheumatoid factor (RF) is an autoantibody directed against the Fc portion of IgG. The most common and clinically significant rheumatoid factor is of the IgM class. In diagnostic tests, a positive RF test typically detects IgM anti-IgG antibodies, which are present in about 70-80% of patients with rheumatoid arthritis (though RF can also occur in other conditions and healthy individuals).

Why not the others?

• a) IgE: Involved in allergic reactions (Type I hypersensitivity), not associated with RF.

• b) IgA: IgA rheumatoid factor exists but is less common and not the primary target in standard RF tests.

• d) IgG: IgG autoantibodies are involved in other autoimmune processes (e.g., in SLE), but the classic RF test detects IgM against IgG.

Type II hypersensitivity reactions are antibody-mediated and involve IgG or IgM antibodies directed against antigens on cell surfaces or extracellular matrix. This leads to cell destruction or dysfunction. Hemolytic anemia (e.g., due to incompatible blood transfusions or autoimmune reactions) is a classic example, where antibodies bind to red blood cells, causing their lysis or phagocytosis.

The other options are incorrect:

• a) Serum sickness: A Type III hypersensitivity reaction caused by immune complex deposition.

• c) Contact dermatitis: A Type IV hypersensitivity (T-cell-mediated delayed) reaction.

• d) Asthma: Typically a Type I hypersensitivity (IgE-mediated) reaction.

DiGeorge syndrome is caused by a deletion on chromosome 22 (22q11.2 deletion syndrome), which leads to defective development of the third and fourth pharyngeal pouches during embryogenesis. This results in thymic hypoplasia or aplasia (underdevelopment or absence of the thymus), causing T-cell deficiency and impaired immune function. Other features include congenital heart defects, hypoparathyroidism, and facial abnormalities.

The other options are incorrect:

• a) SCID (Severe Combined Immunodeficiency): Involves defects in both T and B cells (e.g., due to ADA or IL-7R mutations) but is not primarily due to thymus developmental defects.

• c) Bruton’s disease (X-linked agammaglobulinemia): Caused by a mutation in the BTK gene, leading to absent B cells and antibodies; the thymus is typically normal.

• d) Wiskott-Aldrich syndrome: An X-linked disorder caused by mutations in the WAS gene, affecting T and B cell function and platelet size, but not primarily a thymus developmental defect.

Contact dermatitis is a classic example of type IV (delayed-type) hypersensitivity, which is T-cell-mediated. The tissue damage occurs through the following steps:

1. Sensitization: Haptens (e.g., from poison ivy, nickel) bind to skin proteins and are taken up by antigen-presenting cells.

2. Activation of CD8+ cytotoxic T lymphocytes (CTLs) and CD4+ Th1 cells.

3. Upon re-exposure, these T cells release cytokines (e.g., IFN-γ) that recruit macrophages and other inflammatory cells, and CTLs directly kill skin cells, causing inflammation, blistering, and eczema.

Why not the others?

• a) IgE-mediated mast cell degranulation: Describes type I hypersensitivity (e.g., urticaria, acute allergies), which is rapid and not delayed.

• c) Immune complex deposition: Characteristic of type III hypersensitivity (e.g., serum sickness), involving antigen-antibody complexes.

• d) Activation of the alternative complement pathway: Part of innate immunity or type II/III hypersensitivity, but not central to contact dermatitis.

Hereditary deficiencies of the late complement components (C5, C6, C7, or C8) impair the formation of the membrane attack complex (MAC), which is crucial for lysing gram-negative bacteria, particularly those of the Neisseria genus (e.g., Neisseria meningitidis and Neisseria gonorrhoeae). Individuals with these deficiencies are at significantly increased risk for recurrent systemic Neisseria infections, such as meningococcemia (septicemia) or disseminated gonococcal infection, if exposed to these pathogens.

The other options are incorrect:

• a) Pneumococcal septicemia: More associated with splenic dysfunction or antibody deficiencies (e.g., lack of opsonization), not late complement deficiencies.

• b) Small bowel obstruction: A mechanical or surgical issue, not related to complement deficiencies.

• c) Systemic lupus erythematosus (SLE): Linked to early classical pathway complement deficiencies (C1, C2, C4), not late components.

Pernicious anemia is an autoimmune disorder where the body produces antibodies that attack and destroy the parietal cells in the stomach lining. These cells are responsible for producing intrinsic factor, a protein essential for the absorption of vitamin B12 in the small intestine. Without intrinsic factor, vitamin B12 cannot be absorbed, leading to a deficiency that impairs red blood cell production (megaloblastic anemia) and causes neurological issues.

The other options are incorrect:

• a) Growth hormone: Produced by the pituitary gland; not related to pernicious anemia.

• b) Thyroxine: Produced by the thyroid gland; associated with thyroid disorders like Hashimoto’s or Graves’ disease.

• d) Erythropoietin: Produced by the kidneys; stimulates red blood cell production but is not targeted in pernicious anemia.

A genetic mutation in T-cell receptor genes is a cause of primary immunodeficiency, not secondary. Primary immunodeficiencies are inherited, genetic disorders that are present at birth and result from intrinsic defects in the immune system.

The other options are all classic causes of secondary (acquired) immunodeficiency:

• a) Chemotherapy: A medical treatment that suppresses the immune system.

• b) Malnutrition: An external factor that compromises immune function.

• d) HIV infection: A viral infection that directly targets and destroys immune cells (CD4+ T cells).

Autoimmune polyendocrine syndrome type 3 (APS-3) is defined by the presence of autoimmune thyroiditis (most commonly Hashimoto’s thyroiditis or, less frequently, Graves’ disease) plus at least one other autoimmune disease (e.g., type 1 diabetes, vitiligo, pernicious anemia, celiac disease), but not Addison’s disease or hypoparathyroidism.

• a) Addison’s disease: The presence of Addison’s disease would reclassify the condition as APS-2 (Schmidt’s syndrome).

• c) Vitiligo: This is a common associated condition in APS but is not required for the diagnosis of APS-3.

• d) Hypoparathyroidism: The presence of hypoparathyroidism would reclassify the condition as APS-1 (a rare monogenic disorder).

Human Immunodeficiency Virus (HIV) specifically targets and destroys CD4+ T cells (also known as helper T cells). These cells play a central role in coordinating the adaptive immune response by activating other immune cells (like B cells and cytotoxic T cells). The progressive loss of CD4+ T cells weakens the immune system, leading to acquired immunodeficiency syndrome (AIDS) and increased susceptibility to opportunistic infections and cancers.

The other options are incorrect:

• a) B cells: Not directly destroyed by HIV (though their function is impaired due to lack of T-cell help).

• c) NK cells: Not the primary target of HIV.

• d) Neutrophils: Not directly affected by HIV.

Primary immunodeficiencies affecting B cells (e.g., X-linked agammaglobulinemia, Common Variable Immunodeficiency) lead to impaired antibody production. Antibodies (immunoglobulins) are critical for defense against extracellular bacteria and some viruses. Therefore, patients commonly present with:

• Recurrent sinusitis, otitis media (ear infections), pneumonia, and bronchitis (often caused by bacteria like Streptococcus pneumoniae, Haemophilus influenzae).

Why not the others?

• a) Recurrent viral infections: More characteristic of T-cell deficiencies (e.g., SCID, DiGeorge syndrome), as T cells are vital for antiviral immunity.

• c) Granulomas in multiple organs: Seen in disorders like Chronic Granulomatous Disease (CGD) (a phagocyte defect) or certain T-cell disorders, not primarily B-cell defects.

• d) Delayed-type hypersensitivity reactions: These are mediated by T cells (Type IV hypersensitivity), not B cells or antibodies.

Bruton agammaglobulinemia (X-linked agammaglobulinemia) is caused by a mutation in the BTK gene, which is essential for B-cell development. This results in:

• Severely decreased or absent immunoglobulins (all classes: IgG, IgA, IgM, etc.) due to a block in B-cell maturation.

• Markedly reduced numbers of circulating B cells (not normal, as in option b).

• It is an X-linked recessive disorder, so it primarily affects males (not females, as in option a).

• Patients typically present with recurrent pyogenic (bacterial) infections (e.g., sinopulmonary infections, otitis media) after ~6 months of age, not immediately after birth (as in option d). This is because maternal antibodies provide temporary protection in the first few months.

Chronic active hepatitis (often referring to autoimmune hepatitis) is characterized by high titers of anti-smooth muscle antibodies (ASMA). These antibodies primarily target actin in smooth muscle cells and are a key serological marker for autoimmune hepatitis type 1. Along with anti-nuclear antibodies (ANA), ASMA helps in diagnosing this condition.

The other options are incorrect:

• a) Anti-mitochondrial antibodies: Associated with primary biliary cholangitis (PBC), not hepatitis.

• c) Anti-DNA antibodies: Linked to systemic lupus erythematosus (SLE), not hepatitis.

• d) Anti-parietal cell antibodies: Associated with pernicious anemia and autoimmune gastritis, not hepatitis.

Graves’ disease is a condition where the immune system produces antibodies (specifically, thyroid-stimulating immunoglobulins) that mimic the action of thyroid-stimulating hormone (TSH). These antibodies bind to and activate the TSH receptor on the thyroid gland, leading to overproduction of thyroid hormones (hyperthyroidism). This is a classic example of an autoimmune disease, where the immune system mistakenly attacks the body’s own tissues.

The other options are incorrect because:

• a) Immunodeficiency refers to a weakened or absent immune response (e.g., HIV/AIDS, SCID).

• c) Hypersensitivity type I involves IgE-mediated allergic reactions (e.g., anaphylaxis, hay fever).

• d) Immunoproliferative disorder involves the excessive production of immune cells (e.g., leukemias, lymphomas).

Sézary syndrome is a aggressive form of cutaneous T-cell lymphoma (often considered a leukemic variant of mycosis fungoides) characterized by malignant CD4+ T-cells (helper T-cells) that circulate in the blood and infiltrate the skin. Despite being malignant, these cells often retain their “helper” activity and can stimulate polyclonal immunoglobulin synthesis by B cells, leading to hypergammaglobulinemia.

The other options are incorrect:

• a) Hodgkin lymphoma: Involves Reed-Sternberg cells (derived from B-cells), not T-cells with helper activity.

• b) Acute lymphocytic leukemia (ALL): Often involves precursor B or T cells; malignant T-cells in T-ALL are typically immature and do not retain helper function.

• d) Chronic lymphocytic leukemia (CLL): A B-cell malignancy (not T-cell), where malignant B cells may produce antibodies but are not helped by malignant T-cells.

Wiskott-Aldrich syndrome (WAS) is an X-linked recessive disorder caused by mutations in the WAS gene. It is classically characterized by the triad of:

1. Immunodeficiency (recurrent infections due to defective T and B cell function).

2. Thrombocytopenia (low platelet count with abnormally small platelets, leading to bleeding).

3. Eczema (chronic inflammatory skin condition).

Why not the others?

• a) DiGeorge syndrome: Features thymic hypoplasia, cardiac defects, hypocalcemia, and T-cell deficiency (no eczema or thrombocytopenia).

• b) Ataxia-telangiectasia: Involves cerebellar ataxia, telangiectasias (dilated blood vessels), and immunodeficiency (no thrombocytopenia or eczema).

• d) Severe Combined Immunodeficiency (SCID): Presents with severe defects in T and B cells causing life-threatening infections (no thrombocytopenia or eczema).

Hypersensitivity reactions are exaggerated or inappropriate immune responses to antigens and are classified into four major types (Type I, Type II, Type III, and Type IV) based on the mechanisms involved and the time it takes for the reaction to occur.

• Type I: Immediate hypersensitivity (e.g., allergies, anaphylaxis)

• Type II: Antibody-mediated hypersensitivity (e.g., hemolytic disease of the newborn)

• Type III: Immune complex-mediated hypersensitivity (e.g., systemic lupus erythematosus)

• Type IV: Cell-mediated or delayed-type hypersensitivity (e.g., contact dermatitis, tuberculin skin test reaction)

In systemic rheumatic disorders like systemic lupus erythematosus (SLE), tissue injury is primarily driven by the deposition of immune complexes (antigen-antibody complexes) in various tissues (e.g., kidneys, skin, blood vessels). These complexes activate the complement system and attract inflammatory cells, leading to organ damage. For example, lupus nephritis involves immune complex deposition in the glomeruli, causing inflammation and kidney dysfunction.

The other options are less central or incorrect:

• a) Cytotoxic T cells: Play a role in some autoimmune diseases (e.g., Type 1 diabetes) but are not the primary mechanism in SLE.

• b) IgE activity: Involved in allergic reactions (Type I hypersensitivity), not typical in SLE.

• d) Cytolytic antibodies: Can cause damage in Type II hypersensitivity (e.g., hemolytic anemia), but SLE is characterized by immune complex-mediated (Type III) injury.

Chronic Granulomatous Disease (CGD) is a primary immunodeficiency caused by defects in the NADPH oxidase enzyme complex in phagocytic cells (neutrophils, macrophages). This enzyme is responsible for producing reactive oxygen species (e.g., superoxide) to kill ingested bacteria and fungi. Without it, patients suffer from recurrent bacterial and fungal infections and form granulomas due to persistent inflammation.

The other options are incorrect:

• a) Bruton’s disease (X-linked agammaglobulinemia): Caused by a defect in B-cell development due to mutations in the BTK gene, leading to absent antibodies.

• b) Severe Combined Immunodeficiency (SCID): Caused by various genetic defects affecting both T and B cells (e.g., ADA deficiency, IL-7R mutations).

• d) DiGeorge syndrome: Caused by a deletion on chromosome 22, leading to thymic hypoplasia and T-cell deficiency.

Hyperviscosity syndrome occurs when high levels of paraproteins (monoclonal immunoglobulins) in the blood increase its viscosity, impairing circulation and leading to symptoms like blurred vision, headaches, bleeding, and neurological issues.

Other options:

• IgA, IgG, IgD → may be involved in multiple myeloma, but they rarely cause clinically significant hyperviscosity (except occasionally in IgA myeloma).

The CH50 assay (Total Hemolytic Complement Assay) is a classic test used to evaluate the overall functional activity of the complement system, specifically the classical pathway. It measures the ability of a patient’s serum to lyse antibody-coated sheep red blood cells (RBCs). The “CH50” refers to the dilution of serum that lyses 50% of the RBCs, indicating the integrity of all complement components (C1–C9) in the pathway.

Why not the others?

• a) B cell antibody production: Assessed by other tests (e.g., immunoglobulin levels, specific antibody titers).

• b) T cell mediated immunity: Evaluated via T cell counts, proliferation assays, or skin tests (e.g., DTH reactions).

• d) Phagocyte oxidative burst: Measured by tests like the nitroblue tetrazolium (NBT) test or dihydrorhodamine (DHR) flow cytometry, used for diagnosing Chronic Granulomatous Disease (CGD).

Anti-thyroid peroxidase antibodies (anti-TPO) are autoantibodies that target thyroid peroxidase, an enzyme involved in thyroid hormone production. High titers of anti-TPO are most commonly associated with autoimmune thyroid diseases, particularly:

The other options are incorrect:

• b) Systemic lupus erythematosus (SLE): Linked to antibodies like ANA, anti-dsDNA, or anti-Smith, not anti-TPO.

• c) Chronic hepatitis: Associated with anti-smooth muscle antibodies (autoimmune hepatitis) or viral markers, not anti-TPO.

Myasthenia gravis is an autoimmune disorder where the body produces antibodies that target acetylcholine receptors (AChR) at the neuromuscular junction. These antibodies block, destroy, or interfere with the receptors, preventing acetylcholine from properly binding and transmitting nerve signals to muscles. This leads to muscle weakness and fatigue, which are hallmark symptoms of the disease.

The other options are incorrect:

• a) Multiple sclerosis: Involves antibodies and T-cells attacking the myelin sheath in the central nervous system, not acetylcholine receptors.

• c) Rheumatoid arthritis: Characterized by antibodies targeting joint tissues (e.g., anti-CCP antibodies), not acetylcholine receptors.

• d) Graves’ disease: Involves antibodies targeting the TSH receptor on the thyroid gland (not AChR), leading to hyperthyroidism.

• Secondary immunodeficiencies are acquired later in life and are not inherited. They result from external factors such as infections, medications, malnutrition, or other diseases.

• AIDS (Acquired Immunodeficiency Syndrome) is caused by the HIV virus, which attacks and weakens the immune system. It is a classic example of a secondary immunodeficiency.

The other options are all primary immunodeficiencies (inherited genetic defects):

• a) Chronic granulomatous disease: A genetic disorder where phagocytes cannot effectively kill certain bacteria.

• b) Bruton’s agammaglobulinemia: A genetic disorder leading to a lack of B cells and antibodies.

• d) DiGeorge syndrome: A genetic disorder caused by a deletion on chromosome 22, resulting in thymic hypoplasia and T-cell deficiency.

Type III hypersensitivity is mediated by immune complexes, which are formed when antibodies (IgG or IgM) bind to soluble antigens. These complexes can deposit in tissues, such as blood vessels, kidneys, or joints, and trigger inflammation by activating the complement system and attracting neutrophils. This leads to tissue damage.

The other types involve different mechanisms:

• a) Type I: Mediated by IgE antibodies (e.g., allergies).

• b) Type II: Mediated by IgG or IgM antibodies targeting cell-surface or extracellular matrix antigens (e.g., hemolytic anemia).

• d) Type IV: T-cell-mediated delayed hypersensitivity (e.g., tuberculin skin test, contact dermatitis).

Hashimoto thyroiditis is an organ-specific autoimmune disease where the body produces antibodies (such as anti-thyroid peroxidase and anti-thyroglobulin) that specifically attack the thyroid gland. This leads to inflammation and eventual destruction of the thyroid, causing hypothyroidism.

The other options are not organ-specific:

• a) Wegener granulomatosus (Granulomatosis with polyangiitis): A systemic autoimmune disease affecting blood vessels in multiple organs (e.g., lungs, kidneys).

• b) Rheumatoid arthritis: A systemic autoimmune disease primarily targeting joints but can affect other organs like the skin, eyes, and lungs.

• d) Systemic lupus erythematosus: A systemic autoimmune disease that can affect virtually any organ in the body (e.g., skin, joints, kidneys, heart).

The secondary immune response occurs upon re-exposure to the same antigen and is facilitated by memory B cells generated during the primary response. Key features include:

• Shorter lag phase (not longer, as in option a) due to rapid activation of memory cells.

• Predominant production of IgG (not IgM, as in option b), as class switching has occurred.

• Higher magnitude (titer) of antibodies (option c) because memory cells proliferate quickly and produce more antibodies.

• Longer duration of elevated antibody levels (not shorter, as in option d) due to sustained output from plasma cells.

IL-4 is a key cytokine produced by Th2 cells that drives the class-switching of B cells to produce IgE. This IgE then binds to mast cells and basophils, sensitizing them for future exposure to allergens. Upon re-exposure, cross-linking of IgE triggers degranulation and the release of mediators (e.g., histamine) that cause allergic reactions (e.g., asthma, anaphylaxis).

Why not the others?

• a) IL-2: Promotes T-cell proliferation and activation (not IgE production).

• b) IFN-γ: Produced by Th1 cells; inhibits Th2 responses and IgE class-switching.

• d) TNF-α: Involved in systemic inflammation and immune cell activation but does not directly stimulate IgE production.

Anaphylaxis is the most rapid and severe form of immediate hypersensitivity (Type I), typically occurring within seconds to minutes after exposure to an allergen (e.g., peanuts, penicillin, insect venom). It is mediated by IgE antibodies triggering massive release of histamine and other mediators from mast cells and basophils, leading to life-threatening symptoms like airway obstruction, hypotension, and shock.

The other options are slower or involve different mechanisms:

• a) Transfusion reaction: A Type II hypersensitivity (antibody-mediated cytotoxicity) that can occur within minutes to hours but is not as universally rapid as anaphylaxis.

• c) Contact dermatitis: A Type IV delayed hypersensitivity reaction, developing over 48–72 hours.

• d) Serum sickness: A Type III hypersensitivity (immune complex-mediated) that typically arises 7–14 days after exposure.

Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV). HIV attacks and destroys CD4 T-cells, which are a type of white blood cell that is crucial for the immune system’s function. Over time, this severely weakens the immune system, leading to AIDS.

The other options are not correct because:

• a) Epstein-Barr virus causes infectious mononucleosis and is associated with certain cancers.

• c) Cytomegalovirus is a common virus that can cause illness in people with weakened immune systems.

• d) Hepatitis B virus causes a liver infection, not a systemic failure of the immune system.

The FTA-ABS (Fluorescent Treponemal Antibody Absorption) test is a highly specific confirmatory test for syphilis. A “beaded” pattern of fluorescence is characteristic of genuine treponemal antibodies binding to the Treponema pallidum antigen on the slide. This pattern is not seen in non-syphilitic sera and confirms a true-positive reaction, ruling out biological false positives.

Why not the others?

• b) Biological false-positive reaction: These are typically smooth or non-specific fluorescence patterns (e.g., due to autoimmune diseases like SLE) and are absorbed out in the FTA-ABS procedure.

• c) Late latent syphilis: While FTA-ABS remains positive in late stages, the “beaded” pattern is not stage-specific; it confirms true infection at any stage.

• d) Primary syphilis: The FTA-ABS is positive early, but the pattern is not exclusive to primary syphilis.

Tolerance refers to the immune system’s ability to distinguish between self and non-self antigens, thereby preventing attacks on the body’s own tissues (autoimmunity). This involves mechanisms that delete or suppress self-reactive lymphocytes during their development (central tolerance) and in the periphery (peripheral tolerance).

Why not the others?

• a) Specific immunity: Refers to the adaptive immune response (involving B and T cells) that targets specific antigens, but does not inherently distinguish self from non-self.

• c) Cell-mediated immunity: Involves T cells attacking infected or abnormal cells but does not define the self/non-self recognition principle.

• d) Humoral immunity: Involves antibodies produced by B cells targeting extracellular pathogens, but again, does not encompass the concept of self-tolerance.

Delayed hypersensitivity, also known as type IV hypersensitivity, is a T-cell-mediated immune response that occurs 24–72 hours after exposure to an antigen. It is induced by:

• Contact sensitizers (e.g., inorganic chemicals like nickel, chromium, or organic compounds like poison ivy urushiol). These chemicals act as haptens, binding to skin proteins and triggering a T-cell response, leading to contact dermatitis.

Why not the others?

• b) Transfusion reaction: Typically caused by type II hypersensitivity (antibody-mediated destruction of transfused red blood cells).

• c) Anaphylactic reaction: A type I hypersensitivity (IgE-mediated, rapid onset within minutes).

• d) Bacterial septicemia: A systemic infection that may cause inflammation via innate immune responses or endotoxin-induced shock (not primarily delayed hypersensitivity).

A defect in the adenosine deaminase (ADA) enzyme causes one of the forms of Severe Combined Immunodeficiency (SCID). ADA is essential for breaking down toxic metabolites (like deoxyadenosine) that accumulate in lymphocytes. Without functional ADA, these toxins kill both T cells and B cells, leading to a severe deficiency in adaptive immunity. This results in:

• Profoundly low T-cell and B-cell counts.

• Life-threatening infections from early infancy.

• Autosomal recessive inheritance.

Why not the others?

• a) Bruton’s agammaglobulinemia: Caused by a mutation in the BTK gene, leading to absent B cells and low immunoglobulins (but normal T cells).

• c) Hyper IgM syndrome: Typically caused by mutations in CD40L (on T cells), leading to defective class-switching and elevated IgM (with low IgG/IgA).

• d) Complement deficiency: Involves defects in complement proteins (e.g., C1–C9), not ADA.