Anti-IgA antibodies in individuals with IgA deficiency can cause severe anaphylactic reactions during blood transfusion. These patients lack IgA and may produce anti-IgA antibodies (often of IgG or IgE class). When exposed to IgA-containing blood products (e.g., plasma, platelets, or whole blood), rapid complement activation and anaphylaxis can occur.

• b) IgD: No known anaphylactic reactions associated with anti-IgD.

• c) IgE: Involved in allergic reactions but not typically linked to transfusion anaphylaxis.

• d) IgG: Anti-IgG antibodies (e.g., in TRALI) can cause reactions, but anaphylaxis is specifically tied to anti-IgA in deficient patients.

Fluorescein is a non-enzymatic label commonly used in serologic assays, such as immunofluorescence (e.g., direct or indirect fluorescent antibody tests). It is a fluorochrome that emits light when excited by a specific wavelength, allowing detection without enzymatic reactions.

• a) Horseradish peroxidase, b) Alkaline phosphatase, and d) Beta-galactosidase: These are enzymatic labels used in assays like ELISA, where they catalyze substrates to produce a detectable signal (color change, chemiluminescence).

In hybridoma technology, the goal is to fuse a B-lymphocyte (which produces a specific desired antibody) with a myeloma cell (a cancerous plasma cell that provides immortality and continuous growth). This hybrid cell (myeloma-lymphocyte hybrid) combines the antibody-producing capability of the lymphocyte with the immortality of the myeloma cell, allowing for the continuous production of monoclonal antibodies.

• a) Myeloma-myeloma hybrid: Would not produce specific antibodies.

• c) Lymphocyte-lymphocyte hybrid: Would not be immortal and would die out.

• d) Lymphocyte-granulocyte hybrid: Granulocytes are not involved in antibody production.

Goodpasture syndrome is an autoimmune disorder characterized by rapidly progressive glomerulonephritis and pulmonary hemorrhage. It is directly caused by anti-glomerular basement membrane (anti-GBM) antibodies that attack collagen in the basement membranes of kidneys and lungs.

• a) Anti-mitochondrial antibodies are associated with primary biliary cholangitis.

• b) Anti-smooth muscle antibodies are associated with autoimmune hepatitis.

• d) Anti-parietal cell antibodies are associated with pernicious anemia.

Patients who have received mouse monoclonal antibody therapy (e.g., for cancer or autoimmune diseases) may develop human anti-mouse antibodies (HAMA). In a sandwich ELISA, these HAMAs can bridge the capture and detection antibodies (both often derived from mice), creating a false signal that mimics the presence of the target antigen, leading to a false-positive result.

• b) Mouse virus antibodies: Not a common interferent in ELISA.

• c) Monoclonal gammopathy: May cause general assay interference but is not specific to mouse antibody therapy.

• d) Cross-reactivity with complement: Can cause interference in some immunoassays but is not the primary issue here.

Hemagglutination inhibition (HAI) tests rely on the principle that specific antibodies in a patient’s serum can block (inhibit) a virus’s ability to agglutinate red blood cells. Many viruses (e.g., influenza, measles) have hemagglutinin proteins that bind to and clump RBCs. If the serum contains antibodies against the virus, they will bind to the virus and prevent this agglutination, indicating immunity or recent infection.

• a) Virus causes direct agglutination: This is the basis of the initial hemagglutination assay, not the inhibition test.

• c) Antigen prevents red cell agglutination: Not standard; antibodies are the key inhibitors.

• d) Enzyme reaction inhibits hemagglutination: HAI does not involve enzymes; it is based on antibody-virus interaction.

• Direct immunofluorescence (DIF) involves applying a primary antibody that is directly conjugated to a fluorescent dye (e.g., FITC) to the clinical specimen (e.g., sputum, tissue).

• This labeled primary antibody binds specifically to Legionella pneumophila antigens, allowing visualization under a fluorescence microscope.

• DIF is rapid and specific but requires a separate conjugated antibody for each target. It contrasts with indirect immunofluorescence, which uses a secondary antibody with a fluorescent conjugate.

• HEp-2 cells are used as the substrate because they are rich in nuclear antigens.

• These antigens are unlabeled and fixed to the slide.

• The patient’s serum (containing potential autoantibodies) is applied. If autoantibodies are present, they bind to the nuclear antigens.

• A secondary antibody (labeled with a fluorescent dye) is then added. This antibody is directed against human immunoglobulins (e.g., anti-human IgG).

• The fluorescence is observed under a microscope to detect the presence and pattern of anti-nuclear antibodies.

The combination of a monoclonal spike of IgG (on serum protein electrophoresis), Bence Jones proteinuria (free monoclonal light chains in urine), and bone pain (due to lytic lesions) is classic for multiple myeloma. This is a malignant proliferation of plasma cells in the bone marrow.

• a) Burkitt lymphoma: A B-cell lymphoma associated with EBV, not typically featuring monoclonal IgG spikes or Bence Jones protein.

• b) Bruton disease: X-linked agammaglobulinemia, characterized by a lack of immunoglobulins (no monoclonal spike).

• c) Severe combined immunodeficiency disease (SCID): Involves defects in both T and B cells, leading to severe infections, not monoclonal gammopathies.

Flow cytometry-based serology differs from plate-based assays (like ELISA) in several key ways:

• a) Use of fluorescence detection: Flow cytometry uses fluorescently labeled antibodies to detect targets on cells.

• b) Ability to analyze multiple parameters per cell: It can simultaneously measure multiple markers (e.g., surface proteins, intracellular cytokines) on a cell-by-cell basis.

• c) Quantitative range and sensitivity: It offers high sensitivity and dynamic range for detecting low-abundance antigens and can characterize rare cell populations.

Neutrophils are the primary cellular mediators of immune complex tissue injury (e.g., in Type III hypersensitivity reactions). When immune complexes deposit in tissues, they activate complement and attract neutrophils. Neutrophils release enzymes (e.g., proteases, collagenase) and toxic products that cause inflammation and tissue damage.

• a) Mast cells and c) Basophils: Primarily involved in Type I hypersensitivity (allergic reactions) through histamine release.

• d) Eosinophils: Mainly combat parasites and modulate allergic responses but are not the key effectors in immune complex injury.

diffuse radially from separate wells in a gel (e.g., agarose). They move toward each other and form visible precipitin lines where they meet at optimal proportions (equivalence zone). This allows for the detection and comparison of antigens and antibodies based on the pattern of lines.

• b) Western blot: Involves electrophoretic separation of proteins followed by antibody probing (no diffusion-based movement).

• c) Immunofluorescence: Uses fluorescently labeled antibodies to detect targets in cells/tissues (no gel diffusion).

• d) Agglutination test: Relies on the clumping of particles (e.g., latex or RBCs) by antibodies, not diffusion in gel.

A serial dilution of patient serum is used to determine the antibody titer, which is the highest dilution that still yields a positive reaction (e.g., agglutination or color change). This provides a semi-quantitative measure of antibody concentration in the sample, helping assess immune response strength or disease progression.

• b) To remove interfering substances: Dilution may reduce inhibitors but is not the primary purpose; specific methods like absorption are used.

• c) To check for cross-reactivity: Cross-reactivity is assessed using specific antigens or inhibition assays, not serial dilution.

• d) To incubate at various temperatures: Temperature optimization is done with fixed concentrations, not serial dilutions.

In flow cytometry, forward scatter (FSC) measures the diffraction of light around a cell, which correlates primarily with the cell’s size (diameter or volume). Larger cells cause more light to be scattered in the forward direction.

• a) Cell granularity is measured by side scatter (SSC).

• c) Cell fluorescence is detected by specific fluorescence channels (e.g., FL1, FL2) after staining with fluorescent antibodies or dyes.

• d) Cell charge is not measured by flow cytometry.

MHC Class I molecules in humans are encoded by the HLA-A, HLA-B, and HLA-C genes. These molecules are expressed on nearly all nucleated cells and present endogenous antigens (e.g., viral or tumor peptides) to CD8+ cytotoxic T cells.

• a) Complement: Encoded by MHC Class III genes.

• c) Cytokines: Some cytokines (e.g., TNF) are encoded in the MHC Class III region, but not Class I.

• d) HLA-DP, DQ, DR: These are MHC Class II molecules, expressed primarily on antigen-presenting cells (e.g., macrophages, dendritic cells) and present exogenous antigens to CD4+ helper T cells.

Mature, naïve B lymphocytes co-express IgM and IgD on their surface as B-cell receptors (BCRs). These membrane-bound immunoglobulins allow the B cell to recognize and bind to specific antigens, initiating activation and antibody production.

• a) IgM, IgA: IgA is typically secreted, not commonly surface-bound on circulating B cells.

• b) IgM, IgG: IgG is not typically expressed on naïve B cells; it is produced after class switching.

• d) IgM, IgE: IgE is associated with allergies and parasites; it is not commonly co-expressed with IgM on naïve B cells.

Haptens are small molecules that are antigenic only when bound to a larger carrier protein (e.g., albumin). Alone, they cannot elicit an immune response because they are too small to be recognized by the immune system. However, when attached to a carrier, they become immunogenic and can stimulate antibody production. Classic examples include penicillin (a drug hapten) and poison ivy urushiol (a plant hapten).

• a) Opsonins: Molecules that coat pathogens to enhance phagocytosis (e.g., antibodies, complement proteins).

• b) Adjuvants: Substances added to vaccines to enhance the immune response to an antigen.

• d) Allergens: Antigens that cause allergic reactions (can be complete antigens or haptens, but not all haptens are allergens).

The DPT vaccine (against diphtheria, pertussis, and tetanus) contains antigens that stimulate the recipient’s immune system to produce specific antibodies (humoral immunity) and memory cells. This is an example of active immunity because the body actively generates its own immune response.

• a) Passive immunity involves receiving pre-formed antibodies (e.g., from mother to infant or through immunoglobulin therapy).

• b) Cell-mediated immunity primarily involves T-cells (not antibodies) and is not the primary mechanism for DPT.

• d) Immediate hypersensitivity refers to allergic reactions (e.g., Type I allergies), not vaccination.

A heterogeneous immunoassay requires a physical separation step (e.g., washing) to remove unbound antibodies or antigens before measuring the signal. ELISA (Enzyme-Linked Immunosorbent Assay) is a classic example, as it involves coating a solid phase (e.g., a microplate), adding samples and reagents, and washing between steps to isolate the bound complex.

• b) Radioimmunoassay (RIA) can be either heterogeneous (if separation is needed) or homogeneous, but it is often categorized as heterogeneous when using solid-phase separation.

• c) Fluorescence polarization immunoassay (FPIA) is a homogeneous immunoassay (no separation step required), as the measurement relies on the change in fluorescence polarization when an antibody binds to a small antigen.

• d) Chemiluminescent immunoassay (CLIA) can be either heterogeneous (e.g., with magnetic particle separation) or homogeneous, but many CLIAs are heterogeneous.

IgA is the primary immunoglobulin class found in mucosal secretions such as saliva, tears, breast milk, and respiratory and gastrointestinal secretions. It exists in a dimeric form (with a secretory component) that protects mucosal surfaces from pathogens by preventing their attachment and invasion.

• a) IgG: The most abundant antibody in blood and extracellular fluid, but not dominant in secretions.

• c) IgM: Found mainly in blood; too large to efficiently cross into secretions.

• d) IgD: Primarily functions as a B-cell receptor; negligible in secretions.

In complement fixation tests (CFT), complement is added to the reaction. If an antigen-antibody complex is present, it fixes (consumes) the complement. Then, indicator red blood cells (RBCs) coated with antibodies are added. If complement was fixed, no lysis occurs (positive test). If no complex was present, complement remains and lyses the RBCs (negative test). Thus, complement’s role is to mediate lysis as an indicator.

• a) To precipitate complexes: Precipitation is due to antigen-antibody lattice formation, not complement.

• c) To emit fluorescence: Fluorescence comes from fluorochromes, not complement.

• d) To label antibodies: Antibodies are labeled with enzymes, radioisotopes, or fluorochromes, not complement.

• Direct agglutination involves particles (such as latex beads, red blood cells, or bacteria) that are directly coated with an antigen or antibody to detect the corresponding antibody or antigen in a patient sample. When the target is present, visible clumping (agglutination) occurs.

  • Latex agglutination for rheumatoid factor uses latex particles coated with human IgG. If rheumatoid factor (anti-IgG) is present in the patient’s serum, it binds to the IgG on the latex particles, causing agglutination. This is a direct agglutination test.

• Other options are not direct agglutination:

  • b) ELISA for HBsAg: This is an enzyme-linked immunosorbent assay (ELISA), which uses enzymatic reactions for detection, not agglutination.

  • c) Western blot for Lyme disease: This is an immunoblotting technique that separates proteins by electrophoresis and detects antibodies, not agglutination.

  • d) FTA-ABS for syphilis: This is an indirect immunofluorescence assay, not agglutination.

Acute phase reactants are proteins (e.g., C-reactive protein, fibrinogen, serum amyloid A) whose plasma concentrations increase (or decrease) rapidly in response to inflammation, infection, or tissue injury. This is part of the innate immune response.

• a) Haptens: Small molecules that can elicit an immune response only when attached to a carrier protein.

• b) Opsonins: Molecules (e.g., antibodies, complement) that coat pathogens to enhance phagocytosis.

• d) Chemotaxins: Substances (e.g., chemokines) that attract immune cells to a site of infection or inflammation.

The RPR (Rapid Plasma Reagin) test, like the VDRL, is a non-treponemal test. It detects antibodies (called “reagin”) that are produced in response to lipids released from damaged host cells during syphilis infection. These antibodies react against an artificial antigen made of cardiolipin, lecithin, and cholesterol.

• a) Treponema pallidum: This is the target for treponemal tests (e.g., FTA-ABS, TP-PA).

• c) Reagin: This is the name for the non-specific antibody being detected, not the antigen itself.

• d) Forssman antigen: This is a heterophilic antigen found in various organisms, not related to syphilis testing.

The standard diagnostic algorithm for Lyme disease often uses an ELISA as an initial screening test. If the ELISA result is equivocal (borderline or indeterminate) or positive, it is followed by a Western blot as a confirmatory test. The Western blot detects antibodies against specific Borrelia burgdorferi (the Lyme disease bacterium) antigens, providing higher specificity.

• a) Southern blot is used for DNA analysis.

• c) Northern blot is used for RNA analysis.

• d) Eastern blot is a less common technique, sometimes used for detecting post-translational modifications of proteins, but it is not standard for Lyme disease confirmation.

In a complement fixation test (CFT), complement consumption is detected using an indicator system consisting of sheep red blood cells (RBCs) coated with anti-sheep RBC antibodies (hemolysin). If complement was consumed by the antigen-antibody reaction in the test system, no complement remains to lyse the sheep RBCs → no hemolysis (positive result). If complement was not consumed, it remains available to lyse the sheep RBCs → hemolysis occurs (negative result). Thus, hemolysis (or its absence) is the visual endpoint.

• b) Color change: Used in enzyme-based assays (e.g., ELISA), not CFT.

• c) Fluorescence: Used in immunofluorescence assays.

• d) Turbidity: May indicate agglutination or precipitation, but not the standard readout for CFT.

• Sandwich ELISA is used to measure antigen in patient samples.

• Principle:

  1. Capture antibody is coated on the plate.

  2. Patient antigen binds to the capture antibody.

  3. Detection antibody (enzyme-linked) binds the antigen at a different epitope.

  4. Substrate is added → color change proportional to antigen concentration.

Other formats:

• a) Direct ELISA → detects antigen with enzyme-labeled primary antibody, less commonly used.

• b) Indirect ELISA → detects antibody in patient serum using enzyme-labeled secondary antibody.

• d) Competitive ELISA → can measure antigen or antibody; antigen detection is less straightforward, usually for small molecules.

PCR (Polymerase Chain Reaction) is a target amplification method. It uses enzymes and cycling temperatures to exponentially amplify (make millions of copies of) a specific segment of DNA or RNA, making it detectable.

• a) Western blot separates and detects specific proteins (no amplification of the target itself).

• b) ELISA (Enzyme-Linked Immunosorbent Assay) detects antigens or antibodies using enzyme-linked reactions (signal amplification, not target amplification).

• d) Flow cytometry detects and measures physical and chemical characteristics of cells or particles (no amplification of the target).

In a competitive ELISA, the core principle is competition. A labeled antigen (often enzyme-linked) and the unlabeled antigen (from the patient sample) compete for a limited number of antibody binding sites on the plate. The amount of labeled antigen that binds is inversely proportional to the concentration of the unlabeled antigen in the sample.

• a) Uses two antibodies: This describes a sandwich ELISA (non-competitive), which uses a capture and a detection antibody.

• c) No washing steps required: This is characteristic of homogeneous assays, not ELISA, which is a heterogeneous assay requiring washing steps.

• d) Uses a fluorescent label: While some ELISAs can use fluorescence, the key characteristic is the competition, not the label type. Enzymes (e.g., HRP) are more common for colorimetric detection in traditional ELISA.

The FTA-ABS (Fluorescent Treponemal Antibody Absorption) test is the most specific serological test for diagnosing syphilis. It detects antibodies against Treponema pallidum itself, making it highly specific for confirming syphilis after a positive non-treponemal test (e.g., VDRL or RPR).

• a) VDRL and b) RPR: These are non-treponemal screening tests that detect reagin antibodies. They are sensitive but less specific, as false positives can occur due to other conditions (e.g., autoimmune diseases, infections).

• d) Dark-field microscopy: Directly visualizes T. pallidum from lesions and is specific, but it is not a serological test and is only useful during the primary stage when lesions are present.

In radial immunodiffusion (RID), antigen is placed in a well and allowed to diffuse radially into a gel that contains a uniform concentration of unlabeled antibody. As the antigen diffuses, it forms a precipitin ring with the antibody at equivalence. The ring’s diameter is proportional to the antigen concentration.

• a) Labeled antigen: Used in competitive assays, not RID.

• c) Labeled antibody: Used in techniques like immunofluorescence, not RID.

• d) Unlabeled antigen: The antigen is the unknown sample added to the well, not embedded in the gel

The Western blot for HIV is a confirmatory test that separates HIV proteins (e.g., gp120, gp41, p24) by gel electrophoresis based on molecular weight. These proteins are transferred to a membrane and probed with the patient’s serum. If HIV-specific antibodies are present, they bind to individual viral protein bands, providing a highly specific pattern for confirmation after a positive screening test (e.g., ELISA).

• a) Detecting viral RNA: Done via PCR, not Western blot.

• c) Detecting p24 antigen: Used in early infection screening (antigen tests), but Western blot focuses on antibodies.

• d) Detecting neutralizing antibodies: Involves functional assays (e.g., viral neutralization), not Western blot.

IgM antibodies are the first antibodies produced in response to a new infection. Detecting mumps-specific IgM in serological testing is the standard method to confirm a recent or acute infection. IgG antibodies (a) indicate a past infection or immunity.

The three phases of cancer immunoediting are:

1. Elimination (a): The immune system identifies and destroys cancer cells.

2. Equilibrium (b): A dynamic balance where the immune system controls but cannot fully eliminate the tumor.

3. Escape (c): Cancer cells evade immune destruction, leading to tumor growth.

CH50 measures the total complement activity in the classical pathway. In active SLE, immune complexes consume complement components (e.g., C3, C4), leading to a marked decrease in CH50. This is a key indicator of disease activity and lupus nephritis.

• a) High titers of anti-microsomal antibodies: Associated with autoimmune thyroid diseases (e.g., Hashimoto’s), not SLE.

• b) High titers of anti-smooth muscle antibodies: Associated with autoimmune hepatitis, not SLE.

• d) Decreased serum immunoglobulin levels: Not typical; SLE often features hypergammaglobulinemia due to autoantibody production.

Affinity refers to the strength of the binding interaction between a single antigenic epitope and a single binding site (paratope) on an antibody. It is a quantitative measure of the binding force, often represented by the equilibrium constant (K). High affinity means tight binding between the specific epitope and antibody site.

• b) Overall strength of multiple bonds: This describes avidity, which is the combined strength of multiple binding interactions (e.g., between a multivalent antigen and antibody).

• c) Rate of antibody decay: Related to antibody half-life, not binding strength.

• d) Ability to precipitate antigen: Involves lattice formation in precipitation reactions, influenced by both affinity and avidity.

The equivalence zone (or equivalence point) in precipitation reactions is the optimal ratio of antigen to antibody where both are fully utilized to form large, insoluble complexes (lattices), resulting in maximal visible precipitate. This zone occurs between the prozone (antibody excess) and postzone (antigen excess), where false negatives can happen due to insufficient lattice formation.

• a) Antigen excess: This is the postzone, where precipitate dissolves or fails to form due to too much antigen.

• b) Antibody excess: This is the prozone, where small complexes remain soluble due to too much antibody.

• d) No reaction: Occurs outside the reaction range (e.g., with non-specific antigens/antibodies).

An allograft (or homograft) is a transplant between genetically non-identical members of the same species. A kidney from an unrelated donor fits this definition, as both donor and recipient are human but not genetically identical.

• b) Syngraft (isograft): Transplant between genetically identical individuals (e.g., identical twins).

• c) Autograft: Transplant from one part of a person’s body to another (e.g., skin graft from thigh to burn site).

• d) Xenograft: Transplant between different species (e.g., pig heart to human).

A heterogeneous assay requires a physical separation step to isolate bound from free components before measurement. Radioimmunoassay (RIA) typically involves this separation (e.g., using charcoal, precipitation, or solid-phase antibodies) to measure bound radioactive antigen, making it a classic example of a heterogeneous assay.

• b) Homogeneous FRET assay: Does not require separation; binding is detected by energy transfer changes in solution.

• c) Direct antigen-antibody binding without separation: Refers to homogeneous assays (e.g., FPIA).

• d) Competitive binding without washing: Also characteristic of homogeneous assays (no separation step).

Preventing cross-contamination in serological assays requires multiple precautions:

• a) Using separate pipettes for reagents and samples: Prevents carryover of biological material between different liquids.

• b) Changing gloves between samples: Avoids transferring contaminants or samples via gloves.

• c) Running positive and negative controls: Helps detect contamination or errors but is a quality measure rather than a direct preventive step. However, it is essential for validating assay integrity.

• Rheumatoid factors (RF) are autoantibodies (most commonly IgM, but also IgG, IgA, or IgE) that target the Fc portion of immunoglobulin G (IgG).

• The Fc region is the constant region of the antibody, and binding of RF to IgG can form immune complexes that contribute to inflammation and joint damage in rheumatoid arthritis.

• While RF is associated with rheumatoid arthritis, it can also occur in other autoimmune diseases and infections.

Biological false-positive VDRL reactions occur when the test is positive but the patient does not have syphilis. This is often associated with autoimmune diseases like systemic lupus erythematosus (SLE), where autoantibodies (e.g., antiphospholipid antibodies) react with the cardiolipin antigen used in the VDRL test.

• Gonorrhea (a) does not typically cause a false-positive VDRL.

• Tertiary syphilis (c) would cause a true-positive result.

• While HIV/AIDS (d) can sometimes cause a false-positive, it is less classic and common than the association with SLE.

In indirect hemagglutination (also called passive hemagglutination), red blood cells (RBCs) are coated with a specific antigen (e.g., from a pathogen). These antigen-coated RBCs act as carriers. When mixed with patient serum containing antibodies against that antigen, the antibodies cross-link the RBCs, causing visible agglutination. This amplifies the detection of antibodies due to the large size of RBCs.

• a) Antibody source: RBCs are not the source of antibodies; they are passive carriers.

• c) Enzyme substrate: Relevant in enzyme-linked assays, not hemagglutination.

• d) Signal amplifiers: The RBCs themselves are the visible signal (agglutination), not amplifiers in the molecular sense.

The VDRL (Venereal Disease Research Laboratory) test is primarily used to screen for syphilis by detecting non-treponemal antibodies. It is most commonly performed on serum (blood sample). While it can sometimes be used with cerebrospinal fluid (CSF) for neurosyphilis diagnosis, the primary and recommended sample is serum. Urine and saliva are not standard samples for this test.

The primary advantage of a sandwich ELISA is its ability to directly detect and quantify specific antigens in a sample. This is in contrast to an indirect ELISA, which is designed to detect antibodies. The sandwich format uses two antibodies (capture and detection) that bind to different epitopes on the target antigen, enhancing specificity and sensitivity for antigen measurement.

• a) Uses fewer reagents: Sandwich ELISA typically requires more reagents (two antibodies).

• c) Requires less incubation time: Incubation times are similar between both techniques.

• d) Does not require wash steps: Both sandwich and indirect ELISAs require washing steps to remove unbound components.

In a competitive ELISA, the patient sample (containing unlabeled antigen) competes with a fixed amount of labeled antigen for binding to limited antibody sites. More unlabeled antigen in the sample means it outcompetes the labeled antigen, resulting in less labeled antigen bound and thus a lower signal (e.g., color intensity). The signal is inversely proportional to the antigen concentration in the sample.

• a) Higher signal with more antigen: True for sandwich ELISA, not competitive.

• c) Detects only antibodies: Competitive ELISA can detect both antigens and antibodies, but it is commonly used for small molecules (e.g., hormones, drugs).

• d) No standard curve: A standard curve is essential for quantification in all quantitative ELISAs, including competitive formats.

1. Rubella antigen is coated onto the solid phase (wells of a microplate).

2. The patient’s serum is added. If rubella antibodies are present, they bind to the antigen.

3. A secondary antibody conjugate is added. This is an anti-human IgG antibody conjugated to an enzyme (e.g., horseradish peroxidase or alkaline phosphatase). It binds specifically to the human IgG from the patient.

4. A substrate is added, and the enzyme produces a detectable signal (e.g., color change) proportional to the amount of antibody in the sample.

Thus, the conjugate is not directed against rubella itself but against human immunoglobulins. This makes the assay versatile for detecting antibodies to any pathogen if the corresponding antigen is coated.

Direct immunofluorescence (DIF) is a method where a fluorochrome-labeled antibody binds directly to a pathogen antigen in a patient sample (e.g., tissue section). This allows for immediate visualization of the pathogen under a fluorescence microscope, making it a direct detection technique.

• a) Indirect ELISA: Detects antibodies (indirect detection of pathogen exposure).

• c) Western blot: Separates and detects proteins via antibodies (indirect).

• d) Agglutination inhibition: Detects antigens indirectly by measuring antibody inhibition.

Sensitivity measures a test’s ability to correctly identify individuals who have the disease (true positive rate). A highly sensitive test minimizes false negatives, making it reliable for screening or ruling out disease when negative (as a negative result strongly suggests absence of disease).

• a) Correctly identify those WITHOUT disease: This describes specificity.

• c) Rule in disease when positive: Related to positive predictive value, which depends on prevalence and specificity.

• d) Rule out disease when negative: Related to negative predictive value, which depends on prevalence and sensitivity.

In a direct fluorescent antibody (DFA) test, the primary antibody (specific to the target antigen) is directly conjugated to a fluorochrome (e.g., fluorescein). This labeled antibody binds directly to the antigen in the patient specimen (e.g., tissue section or microorganism), allowing immediate detection under a fluorescence microscope.

• a) Secondary antibody: Used in indirect fluorescent antibody tests for signal amplification.

• b) Antigen: Antigens are not labeled in DFA; they are detected by labeled antibodies.

• d) Enzyme substrate: Relevant to enzyme-based assays (e.g., ELISA), not direct fluorescence.

For bone marrow transplantation, matching the Human Leukocyte Antigen (HLA) system is critically important. HLA antigens (especially HLA-A, HLA-B, HLA-C, and HLA-DR) are key triggers of immune rejection and graft-versus-host disease (GVHD). Close HLA matching between donor and recipient reduces the risk of these complications.

• a) ABO-Rh: While ABO compatibility is important to prevent hemolytic reactions, it is secondary to HLA matching in bone marrow transplants.

• c) CD4/CD8: These are T-cell markers, not antigen systems for matching.

• d) Pi²⁺: Likely a typo or distractor; not a relevant antigen system in transplantation.

CD8+ T lymphocytes (often called cytotoxic T cells or CTLs) directly kill infected cells (e.g., virus-infected cells), tumor cells, or allografts. They recognize antigens presented by MHC Class I molecules and mediate their cytotoxic function through the release of perforin, granzymes, and Fas-FasL interactions.

• a) Delayed-type hypersensitivity: Primarily mediated by CD4+ Th1 cells (not CD8+ T cells).

• b) Regulatory: Typically associated with CD4+ regulatory T cells (Tregs) that express CD25 and FoxP3.

• d) Helper: Carried out by CD4+ T cells, which assist other immune cells (e.g., B cells, macrophages).

Specificity measures a test’s ability to correctly identify individuals who do not have the disease (true negative rate). A highly specific test minimizes false positives, making it reliable for confirming a disease when positive.

• a) Correctly identify those WITH disease: This describes sensitivity.

• c) Detect small quantities of antigen or antibody: Refers to the detection limit or analytical sensitivity, not diagnostic specificity.

• d) Produce reproducible results: Relates to precision or reliability, not specificity.

• The centromere pattern is characterized by discrete speckled staining in the nucleus due to antibodies targeting centromere proteins.

• This pattern is highly specific for CREST syndrome (a subset of systemic sclerosis), which includes Calcinosis, Raynaud’s phenomenon, Esophageal dysmotility, Sclerodactyly, and Telangiectasia.

• While other conditions may show positive ANA results (e.g., SLE with homogeneous or speckled patterns, Sjögren syndrome with speckled pattern), the centromere pattern is a hallmark of CREST syndrome.

C-reactive protein (CRP) is the classic acute phase reactant known for its extremely rapid response time. Its concentration can increase up to 1000-fold (not just 100x) within 24-48 hours after the onset of inflammation, infection, or tissue injury. This makes it a sensitive early marker for acute inflammation.

• a) Alpha-1 antitrypsin: Increases moderately (2-4x) but much slower than CRP.

• b) Haptoglobin: Increases 2-3x during inflammation.

• d) Ceruloplasmin: Increases only slightly (less than 2x) and slowly.

• Indirect fluorescent antibody (IFA) test:

  1. Antigen is immobilized on a slide or well.

  2. Patient’s antibody (primary antibody) binds the antigen if present.

  3. A fluorescent-labeled secondary antibody (anti-human Ig) binds to the patient’s primary antibody.

  4. Fluorescence is visualized under a microscope.

• Direct IF differs: the primary antibody itself is labeled, so no secondary antibody is needed.

Other options:

• a) → describes a direct method incorrectly.

• c & d) → incorrect binding sequences; do not reflect the indirect method.

Natural killer (NK) cells are lymphocytes of the innate immune system that provide nonspecific killing of tumor cells and virus-infected cells. They do not require prior sensitization or antigen-specific recognition (unlike T-cells). NK cells identify and eliminate abnormal cells based on the absence of MHC class I molecules or stress-induced ligands.

• a) Cytotoxic T cells: Kill in an antigen-specific manner (require MHC presentation and prior activation).

• b) Helper T cells: Assist other immune cells but do not directly kill targets.

• d) Antibody and complement: Involve specific antibody recognition (adaptive immunity) for targeted lysis, not nonspecific killing.

Crithidia luciliae is a haemoflagellate substrate used in immunofluorescence assays (IFA) to detect anti-dsDNA (double-stranded DNA) antibodies. Its kinetoplast contains pure, circular dsDNA without other nuclear antigens, making it highly specific for diagnosing systemic lupus erythematosus (SLE).

• a) Rat stomach tissue is used to detect anti-parietal cell antibodies (e.g., in pernicious anemia).

• b) Mouse kidney tissue is sometimes used in ANCA testing but lacks the specificity for anti-dsDNA.

• d) Toxoplasma gondii is unrelated to anti-dsDNA testing; it is associated with toxoplasmosis serology.

This scenario describes anaphylaxis, a severe Type I hypersensitivity reaction that is primarily mediated by IgE antibodies. Upon first exposure to penicillin (or other allergens), susceptible individuals produce IgE specific to the drug. Upon re-exposure, the allergen cross-links IgE bound to mast cells and basophils, triggering rapid release of histamine and other mediators, leading to symptoms like respiratory distress, vomiting, and hives within minutes.

• a) IgG: Involved in Type II/III reactions (e.g., cytotoxic or immune complex-mediated), which are not immediate.

• b) IgA: Associated with mucosal immunity; not involved in anaphylaxis.

• c) IgM: Involved in primary immune responses and complement activation; not key to anaphylaxis.

Selective IgA deficiency is the most common humoral immune deficiency, with an estimated prevalence of 1 in 300 to 1 in 700 people. It is characterized by significantly reduced or absent serum IgA, while other immunoglobulin levels are normal. Many individuals are asymptomatic, but some experience recurrent respiratory or gastrointestinal infections.

• a) Bruton agammaglobulinemia: A rare X-linked disorder involving a complete lack of immunoglobulins.

• b) IgG deficiency: Often refers to subclass deficiencies, which are less common than selective IgA deficiency.

• d) Wiskott-Aldrich syndrome: A rare X-linked disorder affecting both T cells and platelets, not primarily a humoral deficiency.

Th1 cells are a subset of helper T-cells primarily involved in cell-mediated immunity against intracellular pathogens (e.g., viruses, some bacteria). Their classic signature cytokine is Interferon-gamma (IFN-γ), which activates macrophages and other immune cells.

• IL-4 (a) and IL-5 (b) are classic cytokines produced by Th2 cells, which are involved in humoral immunity and responses to parasites.

• IL-10 (d) is an anti-inflammatory cytokine often produced by regulatory T cells (Tregs) and other immune cells to suppress immune responses.

p-Nitrophenyl phosphate (pNPP) is the most common substrate for alkaline phosphatase (AP) in immunoassays. When AP dephosphorylates pNPP, it produces a yellow-colored product (p-nitrophenol) that can be measured spectrophotometrically at 405 nm.

• a) Tetramethylbenzidine (TMB) and b) o-Phenylenediamine (OPD): These are substrates for horseradish peroxidase (HRP), not alkaline phosphatase.

• d) Luminol: Used in chemiluminescent assays with HRP, not typically with AP (which uses dioxetane-based substrates for chemiluminescence).

IgM is the first immunoglobulin isotype produced by the fetus (as early as 20 weeks gestation). Because IgM does not cross the placenta from the mother, elevated IgM levels in a newborn indicate an in-utero infection (e.g., rubella, toxoplasmosis, cytomegalovirus), as the fetus mounted its own immune response.

• a) IgA: Primarily found in mucosal secretions; not significantly produced in utero.

• b) IgG: Crosses the placenta from mother to fetus, providing passive immunity; it reflects the mother’s immune exposure, not the fetus’s.

• d) IgD: Functions mainly as a B-cell receptor; not associated with fetal infections.

• The homogeneous pattern is characterized by uniform staining of the entire nucleus and is strongly associated with antibodies to dsDNA and histone proteins.

• Anti-dsDNA antibodies are highly specific for systemic lupus erythematosus (SLE), and their presence is a key diagnostic criterion.

• While other patterns (like speckled) are associated with various autoantibodies (e.g., anti-Sm, anti-RNP), the homogeneous pattern is classic for anti-dsDNA.

Cryoglobulins (abnormal immunoglobulins that precipitate at cold temperatures) are a common cause of false-positive rheumatoid factor (RF) results. They can interfere with assay measurements, leading to inaccurate positivity. RF is an autoantibody often associated with rheumatoid arthritis but can also be elevated in other conditions.

• b) C1q: Related to complement and immune complexes, not a direct cause of RF false positivity.

• c) Histidine-rich glycoprotein: Involved in coagulation and immune regulation, not typically linked to RF interference.

• d) Aspartame: An artificial sweetener with no known association with RF testing.

• Rheumatoid factor (RF) is an autoantibody directed against the Fc fragment of IgG.

• While RF can be of any immunoglobulin class (IgG, IgA, IgE, or IgM), the most common and routinely detected type is IgM rheumatoid factor.

• Clinical tests for RF often specifically detect IgM RF due to its high prevalence and utility in diagnosing rheumatoid arthritis and other autoimmune conditions.

False positive serological results can be caused by multiple factors:

• a) Cross-reactivity of antibodies: Antibodies may bind to similar epitopes on unrelated antigens (e.g., other pathogens or self-proteins).

• b) High background signal from non-specific binding: Due to factors like hydrophobic interactions or Fc receptor binding.

• c) Improper washing/blocking steps: Inadequate washing fails to remove unbound reagents, and poor blocking allows non-specific attachment to the solid phase.

False-negative results in agglutination tests can arise from multiple factors:

• a) Prozone (antibody excess): Too many antibodies saturate antigen sites, preventing cross-linking and visible clumping.

• b) High antigen concentration (postzone): Excess antigen leads to small, soluble complexes that fail to agglutinate.

• c) Low incubation temperature: Can reduce antibody binding efficiency and slow reaction kinetics, inhibiting agglutination.

• Purpose: In immunofluorescence assays, non-specific binding of the fluorescent-labeled secondary antibody can produce background fluorescence.

• Secondary-only control:

  • Omits the primary antibody.

  • Only the fluorescent secondary antibody is added.

  • Any observed fluorescence indicates non-specific binding or background.

  • Helps distinguish true signal from artifact.

Other options:

• a) Positive antigen control → confirms assay is working.

• b) Negative antibody control → checks for cross-reactivity but does not specifically assess secondary antibody binding.

• d) Heat inactivated control → used in complement or viral assays, not standard for IF background check.

T lymphocytes (T-cells) are key players in cell-mediated immunity and can perform various roles, including:

• a) Cytotoxic cells (CD8+ T-cells that kill infected or cancerous cells),

• b) Helper cells (CD4+ T-cells that assist other immune cells),

• d) Regulatory cells (T-regs that suppress immune responses).

However, T-cells are not phagocytic. Phagocytosis is primarily carried out by innate immune cells like neutrophils, monocytes/macrophages, and dendritic cells. T-cells recognize antigens presented to them by phagocytic cells but do not engulf pathogens themselves.

In latex agglutination tests, the visible clumping (agglutination) occurs when antigens are coated onto latex particles. When these antigen-coated particles are mixed with a patient’s serum containing specific antibodies, the antibodies bind to the antigens, cross-linking the particles and forming visible clumps.

• a) Interaction of soluble antigen with soluble antibody: This describes precipitation reactions (e.g., in gel), not agglutination.

• c) Competition between labeled and unlabeled antigen: This occurs in competitive immunoassays (e.g., ELISA), not agglutination.

• d) Enzymatic color change: This is characteristic of enzyme-linked assays (e.g., ELISA), not agglutination.

Hereditary angioedema (HAE) is an autosomal dominant disorder characterized by deficient or dysfunctional C1 esterase inhibitor (C1-INH). This leads to uncontrolled activation of the complement system (specifically C2 and C4 consumption) and the contact pathway, resulting in episodic swelling of the skin, airways, and gastrointestinal tract.

• a) Decreased C3: Not typical; C3 levels are usually normal in HAE.

• c) Increased C1-INH: Opposite of the actual defect; HAE involves reduced activity.

• d) Increased C2: Not a feature; C2 is consumed due to lack of inhibition, but its activity isn’t increased.

In systemic lupus erythematosus (SLE), tissue injury is primarily caused by the deposition of immune complexes (antigen-antibody complexes) in various tissues (e.g., kidneys, skin, joints). These complexes trigger inflammation by activating complement and attracting neutrophils, leading to organ damage.

• a) Cytotoxic T cells: Play a role in some autoimmune diseases but are not the primary mechanism in SLE.

• b) IgE activity: Involved in allergic reactions, not典型 in SLE.

• d) Cytolytic antibodies: Some autoantibodies in SLE (e.g., anti-RBC) can cause cytolysis, but immune complex deposition is the hallmark of widespread tissue injury.

The speckled pattern on indirect immunofluorescence assay (IFA) for antinuclear antibodies (ANA) is classically associated with antibodies to Sm (Smith) antigen, as well as other antigens like RNP, SSA/Ro, and SSB/La. Anti-Sm antibodies are highly specific for systemic lupus erythematosus (SLE).

• a) Homogeneous: Associated with antibodies to dsDNA or histones (e.g., in SLE or drug-induced lupus).

• c) Nucleolar: Associated with scleroderma (e.g., antibodies to RNA polymerase).

• d) Centromere: Associated with limited cutaneous systemic sclerosis (CREST syndrome).

Primary biliary cirrhosis (PBC), now called primary biliary cholangitis, is characterized by high titers of anti-mitochondrial antibodies (AMA), specifically targeting the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2). This antibody is a hallmark serological marker for PBC, present in ~95% of cases.

• b) Anti-smooth muscle antibodies: Associated with autoimmune hepatitis.

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

• d) Anti-parietal cell antibodies: Linked to pernicious anemia and autoimmune gastritis.

A heterogeneous immunoassay requires a physical separation step (e.g., washing, centrifugation) to isolate bound antigen-antibody complexes from unbound (free) reagents before detection. This reduces background noise and improves specificity. Examples include ELISA and radioimmunoassay (RIA).

• a) Does not require separation: Describes a homogeneous immunoassay (e.g., FPIA).

• c) Uses different labels: Refers to multiplex assays, not the definition of heterogeneity.

• d) Uses antigen/antibody from different species: Common in many assays but not specific to heterogeneous formats.

Phagocytosis is the process of engulfing and destroying pathogens or debris. The primary phagocytic cells (phagocytes) are:

• Neutrophils (first responders to infection),

• Monocytes (which differentiate into macrophages in tissues),

• Eosinophils (weakly phagocytic but primarily combat parasites and modulate allergies).

• Basophils (a) are not phagocytic; they release histamine and other mediators in allergic responses.

• Lymphocytes (c, d) (B-cells, T-cells) are not phagocytic; they are involved in adaptive immunity (antibody production, cell-mediated responses).

The QuantiFERON-TB Gold test is an interferon-gamma release assay (IGRA). It measures the release of IFN-γ (Interferon-gamma) from T-cells in whole blood after stimulation with Mycobacterium tuberculosis-specific antigens. The amount of IFN-γ produced indicates a cellular immune response to M. tuberculosis infection.

• IL-2 (a) is involved in T-cell proliferation but is not measured in this test.

• IL-4 (b) is a cytokine associated with Th2 responses.

• TNF-α (d) is involved in inflammation but is not the cytokine measured for this specific diagnostic assay

• Anti-thyroperoxidase (TPO) antibodies are present in the vast majority (over 90%) of patients with Hashimoto thyroiditis, making them the most sensitive and specific marker for the disease.

• Anti-thyroglobulin (Tg) antibodies are also common but are less sensitive and specific compared to anti-TPO.

• Anti-thyroid-stimulating hormone receptor (TRAb) antibodies are typically associated with Graves’ disease, not Hashimoto thyroiditis.

• Anti-thyroid-stimulating hormone (TSH) antibodies are not clinically significant in this context.

The washing steps in ELISA are critical to remove unbound antibodies, antigens, or other reagents that are not specifically attached to the solid phase. This minimizes non-specific binding and background noise, ensuring that the detected signal (e.g., color change) is specifically due to the target antigen-antibody interaction.

• a) Enhance binding of enzyme label: Washing does not enhance binding; it removes excess label.

• c) Denature non-specific proteins: Washing removes but does not denature proteins; denaturation is not its purpose.

• d) Increase antigen concentration: Washing does not concentrate antigens; it cleanses the well.

In an indirect ELISA, the antigen is coated onto the well to serve as a capture target. When patient serum is added, specific antibodies (if present) bind to the immobilized antigen. This is followed by addition of an enzyme-labeled secondary antibody that recognizes the patient’s antibody, enabling detection.

• a) To capture patient antigen: This describes a sandwich ELISA, where the well is coated with an antibody to capture antigen.

• c) To block non-specific binding: Blocking is a separate step using agents like BSA to cover unused sites.

• d) To generate color directly: The antigen itself does not produce color; signal generation requires enzyme-substrate reaction via the secondary antibody.

agglutinate red blood cells. If the patient has specific antibodies against the virus (e.g., from prior infection or vaccination), these antibodies bind to the viral hemagglutinin proteins, preventing the virus from cross-linking and clumping the red cells. The absence of agglutination indicates a positive result (antibody is present).

• a) Enhances agglutination: Antibodies typically cause agglutination in direct tests, but in HAI, they inhibit it.

• c) Lyses red cells: Complement mediates lysis, not antibodies alone in this context.

• d) Competes with antigen: Relevant in competitive assays (e.g., ELISA), but not the primary mechanism in HAI.

IgE is the immunoglobulin isotype directly associated with immediate hypersensitivity (Type I) reactions, such as allergies, asthma, and anaphylaxis. It binds to high-affinity Fcε receptors on mast cells and basophils. Upon exposure to an allergen, cross-linking of IgE triggers the release of histamine and other mediators, causing rapid allergic symptoms.

• a) IgA: Found in mucosal secretions; protects against infections.

• b) IgM: Involved in primary immune responses; efficient at complement activation.

• c) IgD: Functions mainly as a B-cell receptor; role in hypersensitivity is not defined.

The primary purpose of a blocking agent (e.g., bovine serum albumin, non-fat milk, or casein) in immunoassays is to coat any remaining uncovered surfaces on the solid phase (e.g., microplate wells) after antigen or antibody coating. This prevents non-specific binding of detection antibodies or other reagents to the surface, reducing background noise and false positives.

• a) Enhance antigen-antibody binding: Blocking agents do not improve specific binding; they minimize interference.

• c) Increase signal amplification: Signal amplification is achieved through enzyme-substrate systems or fluorescent labels, not blocking.

• d) Neutralize antigens: Blocking agents are not designed to neutralize antigens; they mask non-specific sites.

A hemagglutination assay (e.g., passive hemagglutination) uses red blood cells coated with a specific antigen. When mixed with patient serum containing antibodies against that antigen, the antibodies cross-link the coated RBCs, causing agglutination (clumping). This indicates a positive antibody detection.

• a) Coombs test: Detects antibodies already bound to RBCs (direct Coombs) or unbound antibodies in serum that can attach to RBCs (indirect Coombs), but does not involve pre-coating RBCs with external antigen.

• c) Neutralization test: Measures antibody ability to neutralize a virus or toxin, often using cell cultures or animal models.

• d) Precipitation assay: Involves soluble antigens and antibodies forming visible precipitates (e.g., in gel), not RBC-based agglutination.

In a complement fixation test (CFT), the absence of hemolysis (no red blood cell lysis) in the indicator system indicates that the complement was fixed (consumed) by an antigen-antibody complex in the initial reaction. This means the test is positive for the specific antibody or antigen being detected.

• b) No antigen-antibody reaction: Would leave complement free, causing hemolysis (negative test).

• c) Excess antigen or d) Excess antibody: Can cause prozone effects but generally still lead to complement fixation if complexes form.

The FTA-ABS (Fluorescent Treponemal Antibody Absorption) test is a treponemal-specific test. It uses whole or sonicated Treponema pallidum (the bacterium that causes syphilis) as the antigen to detect specific anti-treponemal antibodies. This makes it highly specific for syphilis.

• Cardiolipin (a) and Lecithin (d) are components of the non-specific antigen used in non-treponemal tests like VDRL and RPR.

• Reagin (c) is the name for the non-specific antibodies that react in those non-treponemal tests.

The CSF-VDRL (Cerebrospinal Fluid Venereal Disease Research Laboratory) test is the recommended serological test for detecting antibodies in cerebrospinal fluid in cases of suspected neurosyphilis. It is highly specific (though not very sensitive) for neurosyphilis when positive.

• a) Non-treponemal antibody: Refers to tests like RPR or VDRL, but the standard for CSF is specifically the VDRL format adapted for CSF.

• c) FTA-ABS: While sometimes used on CSF, it is less specific for neurosyphilis and not recommended as a first-line test due to potential false positives.

• d) MHA-TP (Microhemagglutination Assay for T. pallidum): Used for serum confirmation, not CSF.

The MHC class III region is located between class I and II on chromosome 6 and contains genes that encode various immune-related proteins, including complement components (e.g., C2, C4, factor B) and cytokines (e.g., TNF-α). Unlike MHC class I and II, which code for antigen-presenting molecules, class III genes are not involved in antigen presentation.

• a) Antigens: These are presented by MHC class I and II molecules, but the genes for the molecules themselves are in class I/II, not class III.

• b) Antibodies: Encoded by genes on different chromosomes (not MHC-linked).

• c) Lymphocytes: Arise from hematopoietic stem cells; their development is not directly coded by MHC genes.

The combination of HBsAg (surface antigen), anti-HBc (core antibody), and often HBeAg (e-antigen) is characteristic of the early acute phase of HBV infection. During this phase:

• HBsAg appears first, indicating active infection.

• Anti-HBc (IgM) rises early and is a marker of acute infection.

• HBeAg indicates high viral replication and infectivity.

This serological profile is seen before the immune system clears the virus and before antibodies like anti-HBs or anti-HBe appear.

• b) Early convalescent phase: HBeAg disappears, and anti-HBe appears; HBsAg may still be present but declining.

• c) Recovery phase: HBsAg and HBeAg are negative; anti-HBs and anti-HBc (IgG) are positive.

• d) Past infection: Only anti-HBc and anti-HBs are present.

In immunoelectrophoresis, antigens are first separated by electrophoresis in a gel. Then, antibodies are added to troughs cut parallel to the electrophoretic separation. As the antigens and antibodies diffuse toward each other, they form precipitin arcs (curved lines) at the point where they meet at optimal proportions. These arcs are visible and indicate the presence of specific antigens.

• b) Hemolysis zones: Seen in complement-related tests, not immunoelectrophoresis.

• c) Agglutination clumps: Characteristic of particle-based tests like latex agglutination.

• d) Color change: Occurs in enzyme-linked assays (e.g., ELISA), not immunoelectrophoresis.

In Western blotting, the first step is to separate antigens (proteins) by electrophoresis (typically SDS-PAGE) based on their molecular weight. The separated proteins are then transferred to a membrane and probed with patient serum to detect specific antibodies binding to individual antigen bands.

• a) Antibody classes: Western blot separates antigens, not antibodies.

• c) Patient’s serum proteins: The sample separated is the antigen mixture (e.g., viral lysate), not the patient’s serum.

• d) Complement components: Not the primary target; Western blot focuses on antigen-antibody interactions.