Immunology and Serology MCQs with Answer and Explanations | Chapter 5

Answer: Slow and sluggish

In the primary immune response, the body encounters an antigen for the first time. This process typically involves a lag period during which the immune system recognizes the antigen, activates B cells to produce antibodies (mainly IgM initially), and generates memory B cells. The primary response is slower and less robust compared to secondary responses.

The other options are incorrect:

• Involves IgG: IgG antibodies are typically produced during the secondary immune response, not the primary immune response.
• Antibody-producing cells. Memory B cell: While memory B cells are generated during the primary immune response, the primary response primarily involves the activation and differentiation of B cells into plasma cells, which then produce antibodies.
• No lag period: This is incorrect. There is typically a lag period in the primary immune response before significant antibody production occurs.

Answer: RT-PCR

During the window period of AIDS, when the virus is present but antibodies may not yet be detectable, the best diagnostic test is RT-PCR (Reverse Transcription Polymerase Chain Reaction). This test directly detects the presence of HIV RNA in the blood, allowing for early diagnosis even before antibody production.

The other options are incorrect:

• ELISA and Western Blot: These antibody tests are not the best option during the window period as they might miss the infection due to the absence of detectable antibodies yet.
• Rapid test: Similar to ELISA and Western Blot, rapid tests also rely on antibody detection and may not be reliable during the window period.

Answer:

The CD4 T cell count is considered the best indicator of HIV prognosis. It reflects the health of the immune system and helps predict the risk of opportunistic infections and disease progression in HIV-positive individuals.

The other options are incorrect:

• CD8 T cell count: While CD8 T cells are involved in the immune response to HIV, the CD4 T cell count is more directly related to HIV prognosis.
• HIV RNA: HIV RNA levels, also known as viral load, are important for monitoring disease progression and response to treatment, but they are not as reliable as CD4 T cell count for predicting long-term prognosis.
• ELISA: ELISA is a diagnostic test used to detect antibodies to HIV, not a prognostic indicator.

Answer: BCG vaccine

The BCG vaccine is a live attenuated vaccine used to prevent tuberculosis (TB). Live attenuated vaccines are created by weakening a virus or bacteria so it can still stimulate the immune system but doesn’t cause full-blown disease.

The other options are incorrect:

• Rabies vaccine: The rabies vaccine is an inactivated vaccine. Inactivated vaccines use killed or weakened viruses that cannot cause infection but can still trigger an immune response.
• Diphtheria toxoid: This is a toxoid vaccine, which means it targets a specific toxin produced by the diphtheria bacteria, not the live bacteria itself.
• Tetanus toxoid: Similar to diphtheria toxoid, tetanus toxoid is another example of a toxoid vaccine targeting the toxin produced by the tetanus bacteria.

Answer: Live attenuated vaccine

Live attenuated vaccines, such as the measles, mumps, and rubella (MMR) vaccine, often require only a single dose for immunization because they contain weakened forms of the virus that can induce a robust immune response with long-lasting immunity.

The other options are incorrect:

• Killed vaccine: Killed vaccines, also known as inactivated vaccines, typically require multiple doses to establish immunity because they contain pathogens that have been inactivated and are less immunogenic than live vaccines.
• Toxoids: These are not live vaccines. Toxoids target inactivated toxins produced by bacteria, not the bacteria itself. Multiple doses might be required to establish immunity depending on the specific toxoid vaccine.
• None of the above: While the number of doses can vary depending on the vaccine and individual factors, live attenuated vaccines generally require only a single dose for healthy individuals.

Answer: AIDS

ELISA (Enzyme-Linked Immunosorbent Assay) is commonly used for the diagnosis of AIDS (Acquired Immunodeficiency Syndrome) by detecting antibodies to HIV (Human Immunodeficiency Virus) in the blood.

The other options are incorrect:

• Night blindness: ELISA is not used for the diagnosis of night blindness. Night blindness is a condition related to vision impairment, often caused by a deficiency in Vitamin A.
• Sickle cell anaemia: ELISA is not used for the diagnosis of sickle cell anaemia. Sickle cell anaemia is a genetic disorder of the red blood cells, and its diagnosis typically involves blood tests, such as hemoglobin electrophoresis or DNA analysis.
• All of the above: This option is incorrect because ELISA is not used for the diagnosis of night blindness or sickle cell anaemia. It is specifically used for the diagnosis of AIDS.

Answer: IgM Anti HBc antobody

The presence of IgM antibodies to the hepatitis B core antigen (IgM Anti HBc antibody) is a reliable marker for acute hepatitis B infection. It indicates recent exposure to the hepatitis B virus.

The other options are incorrect:

• HBsAg: Hepatitis B surface antigen is present in both acute and chronic hepatitis B infections. However, its presence alone does not distinguish between acute and chronic infection.
• HBeAg: Hepatitis B e antigen is a marker of active viral replication. While it can be present in acute hepatitis B infection, it is not the primary marker for diagnosis.
• HBcAg: Hepatitis B core antigen is not typically used for diagnosis as it is not detectable in the blood during routine testing. It is an intracellular antigen found within infected hepatocytes.

Answer: IGA

IgG is the only antibody that crosses the placenta from the mother to the fetus. This transplacental transfer provides passive immunity to the fetus, helping protect the newborn from certain infections during the early months of life.

The other options are incorrect:

• IgA (Immunoglobulin A): IgA antibodies are primarily found in mucosal secretions and do not efficiently cross the placenta.
• IgE (Immunoglobulin E): IgE antibodies are involved in allergic reactions and are not known to significantly cross the placenta.
• IgM (Immunoglobulin M): IgM antibodies are large and do not readily cross the placenta due to their size and structure.

Answer: IgA

IgA is an example of surface Ig, also known as sIg or surface immunoglobulin. It is primarily found on the surface of mucosal epithelial cells, especially in the respiratory and gastrointestinal tracts, where it plays a crucial role in mucosal immunity.

The other options are incorrect:

• IgG: IgG is primarily found in blood and tissue fluids and is not considered a surface immunoglobulin.
• IgE: IgE antibodies are involved in allergic reactions and are not typically found on the surface of cells.
• IgM: IgM antibodies are the first antibodies produced in response to an infection and are primarily found in blood. They are not surface immunoglobulins.

Answer: IgE

Type I hypersensitivity reactions, also known as immediate hypersensitivity reactions, are mediated by IgE antibodies. These reactions occur rapidly after exposure to an allergen and involve the release of inflammatory mediators such as histamine from mast cells and basophils.

The other options are incorrect:

• IgA: IgA is not the primary immunoglobulin involved in type I hypersensitivity reactions.
• IgG: IgG is the most abundant antibody class but is not typically associated with type I hypersensitivity.
• IgM: IgM is primarily involved in the early stages of the immune response and is not a major player in type I hypersensitivity.

Answer: Type I

Wheal and flare is a characteristic feature of Type I hypersensitivity reactions, also known as immediate hypersensitivity reactions. These reactions occur rapidly after exposure to an allergen and involve the release of inflammatory mediators such as histamine, leading to localized swelling (wheal) and redness (flare) at the site of exposure.

The other options are incorrect:

• Type II: Type II hypersensitivity reactions involve antibody-mediated cytotoxicity, damaging target cells through complement activation or antibody-dependent cellular cytotoxicity (ADCC). They don’t typically cause wheal and flare reactions.
• Type IV: Type IV hypersensitivity reactions are cell-mediated hypersensitivity reactions involving T lymphocytes. They manifest as delayed-type reactions, taking hours to days to develop, and wouldn’t present with immediate wheal and flare.
• Type V: Type V hypersensitivity reactions are a less well-defined category involving various mechanisms. They wouldn’t typically cause wheal and flare reactions either.

Answer: Autoimmune hemolytic anemia

Type I hypersensitivity reactions involve the release of inflammatory mediators due to allergen-IgE interactions. Here’s why the other options are considered Type I hypersensitivity.

The other options are incorrect:

• Anaphylaxis: Anaphylaxis is a severe and potentially life-threatening Type I hypersensitivity reaction characterized by systemic mast cell and basophil activation, leading to widespread release of mediators like histamine.
• Extrinsic asthma: Extrinsic asthma, also known as allergic asthma, is a Type I hypersensitivity reaction triggered by exposure to allergens such as pollen, pet dander, or dust mites. It involves airway inflammation, bronchoconstriction, and mucus production.
• Hay fever: Hay fever, or allergic rhinitis, is a Type I hypersensitivity reaction triggered by allergens such as pollen, leading to symptoms like sneezing, runny or stuffy nose, itching, and watery eyes.

Answer: Type IV hypersensitivity

A positive tuberculin test, also known as a Mantoux test or purified protein derivative (PPD) test, is an example of Type IV hypersensitivity. It involves a delayed-type hypersensitivity reaction mediated by T cells, specifically CD4+ T cells, in response to the intradermal injection of tuberculin antigen derived from Mycobacterium tuberculosis.

The other options are incorrect:

• Type I hypersensitivity: This is not the case as a tuberculin skin test doesn’t involve immediate allergic reactions.
• Type II hypersensitivity: This type wouldn’t be involved as the test doesn’t target specific cells for destruction.
• Type III hypersensitivity: This type wouldn’t be triggered by a tuberculin skin test, which relies on T cell response.

Answer: Second set rejection

The phenomenon where a second skin graft from the same donor is rejected more rapidly and vigorously than the first graft is known as second set rejection. This accelerated rejection is due to the memory response of the recipient’s immune system, which has been sensitized by the first graft.

The other options are incorrect:

• First-set rejection: This wouldn’t occur for a second graft from the same donor because the immune system already has memory T cells specific to the donor’s antigens.
• Both: While the rejection will be faster and stronger, it wouldn’t be exactly like a first-set rejection due to the involvement of memory T cells.
• None: The presence of memory T cells from the first exposure will lead to a definite rejection of the second skin graft.

Answer: Isograft

Isograft refers to transplantation between individuals with identical genetic constitution, such as identical twins. Since the genetic makeup is the same, there is minimal risk of rejection, making isografts the most compatible type of transplantation.

The other options are incorrect:

• Autograft: An autograft refers to a transplant where the recipient’s own tissue is used in a different location within the body. For example, a skin graft taken from one area of the body and used in another.
• Allograft: Allograft refers to transplantation between genetically non-identical individuals of the same species. It is the most common type of transplantation and is associated with varying degrees of immune rejection.
• Xenograft: A xenograft refers to a transplant between individuals of different species. For example, a pig heart transplant into a human would be considered a xenograft. Xenografts are very challenging due to significant incompatibility issues.

Answer:

IgE is the immunoglobulin that is inactivated at high temperatures. Heat denaturation causes IgE molecules to lose their biological activity. This property makes it useful for certain laboratory techniques where IgE interference needs to be eliminated.

The other options are incorrect:

• IgG: IgG antibodies are relatively stable and are not typically inactivated at high temperatures. They can withstand moderate heat without losing their activity.
• IgA: IgA antibodies are resistant to heat and remain stable at high temperatures. They are often found in bodily secretions like saliva and tears.
• IgM: IgM antibodies are generally stable at high temperatures and are not typically inactivated by heat. They can function effectively under various conditions.

Answer: Glycoproteins

Immunoglobulins, also known as antibodies, are glycoproteins. They consist of a protein component and a carbohydrate component (glycan) attached to the protein portion. This glycosylation is essential for various functions of immunoglobulins, including antigen recognition and immune signaling.

The other options are incorrect:

• Glycolipids: These are molecules where sugars are attached to lipids (fats). Immunoglobulins don’t have this structure.
• Polypeptides: Polypeptides are simply chains of amino acids linked together. While immunoglobulins are made up of protein chains, they are folded and have additional features like glycosylation, qualifying them as glycoproteins.
• Proteoglycans: These are complex molecules with a core protein linked to glycosaminoglycan chains (long sugar chains). Immunoglobulins have a simpler structure with protein chains and attached sugars, but not the extensive glycosaminoglycan chains seen in proteoglycans.

Answer: Natural passive immunity

Natural passive immunity refers to the immunity passed from mother to child through the placenta or breast milk.

The other options are incorrect:

• Innate immunity: This is the body’s non-specific first line of defense, present at birth and not dependent on prior exposure. (e.g., skin, mucous membranes)
• Natural active immunity: This immunity develops when the body is exposed to a pathogen and produces its own antibodies in response. (e.g., recovering from chickenpox)
• Artificial active immunity: This immunity is acquired through vaccination, where a weakened or inactive form of a pathogen is introduced to stimulate the body’s immune response. (e.g., tetanus vaccination)

Answer: Artificial passive immunity

Anti-D given to an Rh-negative pregnant woman with an Rh-positive fetus provides temporary immunity by introducing pre-formed antibodies against Rh antigen into the bloodstream, preventing the woman’s immune system from producing its own antibodies that could harm the fetus.

The other options are incorrect:

• Artificial active immunity: This is acquired through vaccination, stimulating the body to produce its own antibodies.
• Natural passive immunity: This is immunity passed from mother to child through placenta or breast milk.
• Adoptive immunity: This involves the transfer of immune cells or antibodies from a donor to a recipient to provide temporary immunity.

Answer: Type Il hypersensitivity

Graves’ disease is an autoimmune disorder where the body’s immune system mistakenly attacks the thyroid gland, leading to overproduction of thyroid hormones. This type of response involves antibodies binding to specific cell surface antigens, which is characteristic of Type II hypersensitivity reactions.

The other options are incorrect:

• Type I hypersensitivity: This involves IgE-mediated reactions, such as allergies.
• Type III hypersensitivity: This involves immune complex-mediated reactions, such as autoimmune diseases like systemic lupus erythematosus.
• Type IV hypersensitivity: This involves delayed-type hypersensitivity reactions mediated by T cells, often seen in conditions like contact dermatitis or organ transplant rejection.

Answer: Type ii hypersensitivity

Rh incompatibility involves a Type II hypersensitivity reaction where maternal antibodies (IgG) against the Rh factor antigen on fetal red blood cells lead to their destruction, potentially causing hemolytic disease of the newborn.

The other options are incorrect:

• Type I hypersensitivity: This involves immediate allergic reactions mediated by IgE antibodies, such as in allergic rhinitis or anaphylaxis.
• Type III hypersensitivity: This involves immune complex-mediated reactions, not directly related to Rh incompatibility.
• Type IV hypersensitivity: This involves delayed-type hypersensitivity reactions mediated by T cells, not characteristic of Rh incompatibility.

Answer: Any of the above

Antigens can be composed of various types of biomolecules.

• Lipids: These are fatty molecules found in the membranes of viruses and some bacteria.
• Polysaccharides: These are complex sugars found on the cell walls of bacteria and fungi.
• Proteins: These are the most common type of antigen. They can be found on the surface of viruses, bacteria, and other pathogens, as well as on toxins and other foreign molecules.

Answer: Tuberculin test

The tuberculin skin test is a type of diagnostic test that relies on cell-mediated immunity to detect prior exposure to tuberculosis bacteria. T cells recognize the injected tuberculin (purified protein derivative, PPD) and trigger an inflammatory response, causing a bump to form at the injection site within 48-72 hours.

The other options are incorrect:

• Transfusion reactions: These can be caused by incompatibility between blood types, leading to antibody-mediated reactions (Type II hypersensitivity) rather than cell-mediated immunity.
• Arthus reactions: These are a type of immune complex reaction (Type III hypersensitivity) involving antibodies and immune complexes causing inflammation.
• Type II Lepra reaction (Leper reaction): This is a cell-mediated immune response (Type IV) against Mycobacterium leprae, the bacteria causing leprosy. However, it’s not a diagnostic test but a pathological reaction in leprosy patients.

Answer: Type IV hypersensitivity

The tuberculin test is an example of Type IV hypersensitivity, also known as delayed-type hypersensitivity (DTH). It involves a T cell-mediated immune response to antigens derived from Mycobacterium tuberculosis, resulting in inflammation at the site of injection in individuals previously exposed to the bacteria

The other options are incorrect:

• Type I hypersensitivity: This involves immediate allergic reactions mediated by IgE antibodies, not characteristic of the tuberculin test.
• Type II hypersensitivity: This involves antibody-mediated reactions, not characteristic of the tuberculin test.
• Type III hypersensitivity: This involves immune complex-mediated reactions, not characteristic of the tuberculin test.

Answer: 250-500 units

The recommended dose of human tetanus immunoglobulin (Ig) for post-exposure prophylaxis is typically within the range of 250-500 units. This provides passive immunity against tetanus toxin to individuals at risk of tetanus infection.

The other options are incorrect:

• 500-750 units: This exceeds the typical recommended dose range for post-exposure prophylaxis.
• 750-1000 units: This exceeds the typical recommended dose range for post-exposure prophylaxis.
• 1000-1200 units: This exceeds the typical recommended dose range for post-exposure prophylaxis.

Answer: Syphilis

The VDRL (Venereal Disease Research Laboratory) test is a screening tool specifically designed to detect antibodies produced by the body in response to a syphilis infection.

The other options are incorrect:

• HIV: HIV (Human Immunodeficiency Virus) is diagnosed using specific tests such as ELISA and Western blot.
• Typhoid: Typhoid is diagnosed through blood culture or specific serological tests for Salmonella typhi bacteria.
• Tuberculosis: Tuberculosis is diagnosed through tests like sputum culture, chest X-ray, and tuberculin skin test (Mantoux test).

Answer: IgA

Immunoglobulin A (IgA) is the primary immunoglobulin responsible for local immunity at mucosal surfaces. It’s found in high concentrations in tears, saliva, mucus, and breast milk, acting as a first line of defense against pathogens trying to enter the body through these openings.

The other options are incorrect:

• IgM: IgM is the first antibody produced during a primary immune response but is not specifically associated with local immunity. It’s found in blood plasma and plays a role in agglutination (clumping) of pathogens.
• IgD: The exact function of IgD is still under investigation, but it’s believed to play a minor role in B cell activation and is not a major contributor to local immunity.
• IgG: IgG is the most abundant antibody in the blood and provides systemic immunity throughout the body. While it can be found in some secretions, IgA is the dominant immunoglobulin for localized defense.

Answer: Syphilis

RPR (Rapid Plasma Reagin) and VDRL (Venereal Disease Research Laboratory) tests are specifically designed to detect antibodies produced by the body in response to a syphilis infection.

The other options are incorrect:

• AIDS: AIDS (Acquired Immunodeficiency Syndrome) is the late stage of HIV (Human Immunodeficiency Virus) infection. While some tests can identify HIV antibodies, RPR and VDRL are not used for HIV diagnosis.
• Hepatitis: Hepatitis refers to inflammation of the liver, often caused by viruses. There are different types of hepatitis (A, B, C, etc.), each requiring specific tests for diagnosis. RPR and VDRL are not used for hepatitis testing.
• Malaria: Malaria is a parasitic infection transmitted by mosquitoes. It requires a different diagnostic approach involving blood tests to detect the parasite itself or antibodies against it. RPR and VDRL are not used for malaria diagnosis.

Answer: Coombs test

The Coombs test is specifically designed to detect incomplete antibodies, which are antibodies unable to clump (agglutinate) cells on their own. The test uses an antiglobulin reagent that binds to the Fc portion of the incomplete antibody attached to the target cell, causing visible clumping and confirming the presence of the antibody.

The other options are incorrect:

• Electrophoresis: Electrophoresis is a technique used to separate molecules based on their size and electrical charge. While it can be used to analyze proteins like antibodies, it doesn’t directly detect incomplete antibodies.
• Agglutination test (unless specified): While agglutination tests are commonly used in antibody detection, they typically rely on the ability of complete antibodies to cause clumping of cells or particles. They might not be sensitive enough to detect incomplete antibodies without additional steps like the Coombs test.
• VDRL test: The VDRL (Venereal Disease Research Laboratory) test is a specific type of agglutination test used to detect antibodies against syphilis. While it detects antibodies, it’s not designed for general incomplete antibody detection.

Answer: IL-3

IL-3 is a multilineage cytokine, meaning it acts on multiple types of precursor cells (lineages) in the hematopoietic system. It stimulates the growth and differentiation of various blood cell types, including red blood cells (erythrocytes), white blood cells (myeloid and lymphoid lineages), and megakaryocytes (platelet precursors).

The other options are incorrect:

• IL-1 (Interleukin-1): While IL-1 is involved in the immune response, it primarily functions as a pro-inflammatory cytokine and doesn’t directly stimulate the development of multiple blood cell lineages.
• IL-2 (Interleukin-2): IL-2 is crucial for T lymphocyte activation and proliferation but doesn’t have a direct multilineage effect on hematopoietic stem cells.
• IL-4 (Interleukin-4): IL-4 plays a role in B lymphocyte differentiation and antibody production, but its influence is limited to the lymphoid lineage, not considered multilineage.

Answer: Eosinophils

Allergic reactions often lead to an increase in the presence of eosinophils. Eosinophils are a type of white blood cell that plays a key role in allergic responses, particularly in combating parasites and modulating inflammation associated with allergic diseases like asthma and allergic rhinitis.

The other options are incorrect:

• Lymphocytes: Lymphocytes are involved in specific immune responses and are not typically associated with the immediate response to allergic reactions.
• Neutrophils: Neutrophils are the most abundant type of white blood cell and are primarily involved in the nonspecific immune response to bacterial infections. While they may increase during inflammation, they are not specifically associated with allergic reactions.
• Monocytes: Monocytes are a type of white blood cell that can differentiate into macrophages and dendritic cells. They are involved in the immune response to pathogens and tissue repair but are not specifically associated with allergic reactions.

Answer: IL-3

IL-3 is a multilineage cytokine, meaning it acts on and stimulates the growth and differentiation of various precursor cells (lineages) in the hematopoietic system. This translates to supporting the development of multiple types of mature blood cells, including:

The other options are incorrect:

• IL-1: IL-1 is a pro-inflammatory cytokine involved in various immune responses and inflammation.
• IL-2: IL-2 is primarily involved in the growth and proliferation of T cells.
• IL-4: IL-4 is a cytokine that plays a key role in regulating immune responses, particularly promoting Th2 cell differentiation and B cell activation.

Answer: IgM

IgM is the first antibody produced during the primary immune response.

The other options are incorrect:

• IgG: IgG is the most abundant antibody in the blood and is involved in secondary immune responses.
• IgA: IgA is primarily found in mucosal areas and secretions, providing localized immunity.
• IgD: IgD is primarily found on the surface of B cells and its exact function is not fully understood, but it may play a role in B cell activation.

Answer: Edward Jenner

Edward Jenner, an 18th-century English physician, is credited with discovering the first successful vaccine – the smallpox vaccine – in 1796.

The other options are incorrect:

• Robert Koch: A German microbiologist known for identifying the causative agents of various diseases like tuberculosis and anthrax.
• Louis Pasteur: A French microbiologist known for his work on vaccines, including the rabies vaccine, and pasteurization.
• Lister: Joseph Lister, a British surgeon, is a pioneer of antiseptic surgery, not vaccine development.

Answer: Metchnikof

Metchnikoff discovered phagocytosis, the process by which certain cells engulf and digest foreign particles.

The other options are incorrect:

• Louis Pasteur: Known for his contributions to microbiology, including pasteurization and the germ theory of disease, but not credited with discovering phagocytosis.
• Alexander Fleming: Discovered penicillin, the first antibiotic, but not associated with the discovery of phagocytosis.
• Robert Koch: Known for his work on germ theory and discovering the causative agents of several diseases, but not credited with discovering phagocytosis.

Answer: Anaphylaxis

Anaphylaxis is a severe, potentially life-threatening hypersensitivity reaction that can occur within minutes of exposure to an allergen, even on first encounter. The initial exposure acts as both the sensitizing and shocking dose, triggering a rapid and massive immune response.

The other options are incorrect:

• Arthus reaction: This type of hypersensitivity typically develops after repeated exposure to an antigen. It involves immune complex formation and tissue damage, taking hours to days to manifest.
• Serum sickness: Similar to Arthus reactions, serum sickness usually develops after exposure to foreign proteins, often from medications, after a few days or weeks. It’s not triggered by a single initial dose.
• Contact dermatitis: This type of hypersensitivity reaction involves a T-cell mediated immune response to direct skin contact with an allergen. It typically takes days of repeated or prolonged contact to develop.

Answer: Severe reaction following the injection of protein solution in a sensitized individual

Anaphylaxis is a hypersensitive immune response triggered by exposure to an allergen (often a protein) in someone already sensitized to it. This means they’ve had a prior exposure that primed their immune system to react excessively upon subsequent encounters.

The other options are incorrect:

• Severe reaction following primary injection of Protein solutions: This wouldn’t be anaphylaxis as anaphylaxis requires prior sensitization.
• State of immunity developed by repeated injections of any foreign substance: This describes immunization, the process of developing immunity through controlled exposure to antigens.
• Severe reaction resulting from sensitivity to common allergens: While common allergens can trigger anaphylaxis, the key point is the prior sensitization that leads to the severe reaction.

Answer: NK Cells

NK (Natural Killer) cells are components of innate immunity that are active against viral cells.

The other options are incorrect:

• Cytotoxic T cells: These are part of the adaptive immune system, requiring prior exposure to develop a specific response against a virus.
• B cells: B cells are also part of the adaptive immune system and produce antibodies specific to pathogens.
• Memory B cells: While memory B cells are part of the adaptive immune system and can respond faster upon re-exposure, they aren’t directly involved in the initial innate immune response against a new virus.

Answer: IL-8

IL-8 is a chemokine, a type of cytokine that signals nearby immune cells to migrate towards a site of infection or inflammation. It plays a crucial role in attracting various immune cells, including neutrophils, to fight pathogens and initiate the immune response.

The other options are incorrect:

• IL-1: IL-1 is a pro-inflammatory cytokine involved in various immune responses and inflammation, but it’s not primarily a chemotactic cytokine.
• IL-6: IL-6 is a pleiotropic cytokine involved in inflammation, immune response, and hematopoiesis, but it’s not primarily a chemotactic cytokine.
• TNF (Tumor Necrosis Factor): While TNF is a cytokine involved in inflammation and immune cell activation, it doesn’t primarily function in chemotaxis (cell migration).

Answer: Active

A negative phase is a temporary period of decreased immunity that can occur following vaccination or exposure to a weakened pathogen during active immunity development. The body’s immune system is busy learning to recognize and respond to the new threat, and during this time, it may be less effective at fighting off other infections.

The other options are incorrect:

• Passive: Passive immunity involves receiving antibodies directly from another source, so there’s no need for the body to develop its own immune response and no negative phase.
• Herd: Herd immunity refers to the indirect protection provided to a population when a high percentage of individuals are immune to a specific infection. It doesn’t involve individual immune response stages like a negative phase.
• Local: Local immunity refers to the immune response concentrated at a specific site, like the mucosal surfaces. It doesn’t encompass a negative phase as a general characteristic.

Answer: Arthus phenomenon

The described scenario involves injecting horse serum (foreign protein) into a rabbit, followed by a second injection at a different site weeks later. This delayed, localized reaction with tissue damage is characteristic of an Arthus phenomenon.

The other options are incorrect:

• Atopy: Atopy refers to a genetic predisposition to develop allergic diseases like hay fever or eczema. It wouldn’t describe the specific localized reaction in this case.
• Anaphylaxis: Anaphylaxis is a severe, systemic allergic reaction that can occur rapidly after exposure to an allergen. While it can involve skin reactions, it wouldn’t be limited to the injection site weeks later.
• Serum sickness: Serum sickness is similar to Arthus reactions but typically develops after a longer period (days to weeks) and involves more widespread symptoms beyond just the injection site.

Answer: Cytotoxic / cytolytic type

Type II hypersensitivity involves cytotoxic or cytolytic reactions mediated by antibodies directed against specific target cells or tissues.

The other options are incorrect:

• Anaphylaxis: This is a severe form of type I hypersensitivity, involving IgE antibodies and mast cell degranulation, causing widespread allergic reactions.
• Delayed hypersensitivity: This refers to type IV hypersensitivity, mediated by T cells and causing a delayed inflammatory response after several days.
• Immune complex mediated: This describes type III hypersensitivity, where immune complexes (antigen-antibody) deposit in tissues, triggering inflammation and tissue damage.

Answer: CD7+ Pro-T-Cells

During T cell development in the thymus, the earliest identifiable thymocytes (immature T cells) express CD7 on their surface. These are called CD7+ Pro-T cells. They haven’t yet committed to the CD4+ or CD8+ lineage and undergo further maturation steps to become mature T lymphocytes.

The other options are incorrect:

• CD5+ Pre-T-Cells: While CD5 is expressed on pre-T cells, they are not the earliest identifiable cells of T-cell lineage during maturation.
• CD8+ Pro-T-Cells: CD8 is expressed on mature cytotoxic T cells, not on the earliest identifiable cells during T-cell maturation.
• CD8+ Pre-T-Cells: CD8 is not typically expressed at the pre-T cell stage of development.

Answer: IgE

IgE (Immunoglobulin E) is primarily involved in allergic reactions and doesn’t activate the classical complement pathway.

The other options are incorrect:

• IgM: IgM is known to fix complement efficiently, especially in its pentameric form.
• IgA: IgA can fix complement, particularly in certain circumstances, although it’s less efficient compared to IgM and IgG.
• IgG: IgG is known to fix complement efficiently, especially through its classical pathway activation.

Answer: IgE

Reaginic antibodies are a type of antibody associated with allergic reactions. They bind to the Fc receptors on mast cells and basophils, triggering the release of inflammatory mediators when exposed to allergens. IgE is the primary immunoglobulin class known to function as a reaginic antibody.

The other options are incorrect:

• IgG: IgG is not typically considered a reaginic antibody; it’s involved in various immune responses, including opsonization and complement activation.
• IgA: IgA is primarily found in mucosal areas and secretions, providing localized immunity, but it’s not typically considered a reaginic antibody.
• IgD: IgD is primarily found on the surface of B cells and its exact function is not fully understood, but it’s not typically considered a reaginic antibody.

Answer: IgG

The secondary response is primarily mediated by IgG antibodies.

The other options are incorrect:

• IgA: IgA is primarily found in mucosal areas and secretions, and while it can contribute to immune responses, it’s not typically the primary mediator of the secondary response.
• IgM: IgM antibodies are produced during the primary immune response, and their levels decline before the secondary response where IgG predominates.
• IgE: IgE antibodies are involved in allergic responses and parasitic infections but are not typically the primary mediators of the secondary immune response.

Answer: L chain

Light chains (L chains) of an immunoglobulin molecule have a molecular weight of approximately 25,000 Daltons (Da). This falls within the range specified (around 50,000 Da).

The other options are incorrect:

• Secretory piece: A component added to some immunoglobulins during their transport across mucosal epithelium, not primarily determined by its molecular weight.
• H chain: The heavy chain of an immunoglobulin molecule can have a much larger molecular weight, typically over 50,000 Daltons.
• J piece: A part of the structure of antibodies, but it’s not primarily defined by its molecular weight and is not typically up to 50,000 Daltons.

Answer: T cells

Killer (cytotoxic T lymphocytes) and helper T cells are both subpopulations of lymphocytes that originate in the bone marrow and mature in the thymus. They play crucial roles in cell-mediated immunity, a branch of the adaptive immune response.

The other options are incorrect:

• B cells: B cells are another type of lymphocyte that mature in the bone marrow and are responsible for producing antibodies, a key component of humoral immunity.
• Monocytes: Monocytes are a type of white blood cell that can differentiate into macrophages or dendritic cells, but they are not specifically referred to as killer or helper cells.
• Macrophages: Macrophages are phagocytic cells involved in innate immunity and antigen presentation, but they are not specifically referred to as killer or helper cells.

Answer: Phytohaemagglutinin

Phytohaemagglutinin (PHA) is a lectin, a type of molecule that binds to sugars on cell surfaces. In the context of T cells, PHA can stimulate them to proliferate (multiply) non-specifically. This is often used in laboratory settings to study T cell function.

The other options are incorrect:

• Macrolin: Macrolide antibiotics are a class of drugs used to treat bacterial infections. They wouldn’t stimulate T cell multiplication.
• Heat: While excessive heat can damage cells, it’s not a normal way to stimulate T cell multiplication.
• Bovine serum: Bovine serum is animal serum, which might contain various components. It may not specifically stimulate T cell multiplication and could have unpredictable effects.

Answer: Eosin

In blood banking, common antisera used for blood grouping are color-coded to prevent laboratory errors. The specific dyes used for these reagents are:

• Anti-A: Colored blue (typically using Methylene blue, Patent blue, or Trypan blue).
• Anti-B: Colored yellow (typically using Acriflavin or Tartrazine).
• Anti-AB: Traditionally colored pink using Eosin (though many modern commercial monoclonal Anti-AB reagents are colorless).
• Anti-D (Rh): Typically colorless.

Based on the options provided in the context of standard laboratory examinations and textbooks:

• Methylene blue is for Anti-A.
• Acriflavin is for Anti-B.
• Eosin is for Anti-AB.
• Safranin is a common counterstain in microbiology (e.g., Gram staining) but is not typically used for blood grouping antisera.