Immunology Practice Questions

Q: Which of the following streptococcal antigens cross-reacts with synovial fluid?

Capsular hyaluronic acid. ### Explanation The correct answer is **C. Capsular hyaluronic acid**. **Mechanism of Cross-reactivity:** *Streptococcus pyogenes* (Group A Streptococcus) possesses a capsule composed of **hyaluronic acid**. This polysaccharide is chemically identical to the hyaluronic acid found in human connective tissues, particularly in **synovial fluid** and joint cartilage. Because the body recognizes hyaluronic acid as "self," the capsule is non-immunogenic (it does not elicit an immune response), allowing the bacteria to evade phagocytosis. However, in the context of post-streptococcal sequelae like Acute Rheumatic Fever, this molecular mimicry is a key factor in tissue targeting. **Analysis of Incorrect Options:** * **A. Carbohydrate (Group A):** This is the C-substance used for Lancefield grouping. It cross-reacts specifically with the **cardiac valves** (specifically bovine heart valves and human glycoprotein). * **B. Cell wall protein (M-protein):** This is the major virulence factor. It cross-reacts with **cardiac myosin** and sarcolemmal membrane proteins, leading to rheumatic carditis. * **D. Peptidoglycan:** While it provides structural integrity and has some pyrogenic properties, it is not the primary antigen associated with synovial cross-reactivity. **NEET-PG High-Yield Pearls:** * **Molecular Mimicry Table:** * **Capsule:** Synovial fluid/Joints. * **M-Protein:** Myocardium (Myosin). * **Group A Carbohydrate:** Cardiac valves. * **Cytoplasmic Membrane:** Subthalamic/Caudate nuclei (linked to Sydenham’s Cholera). * **Protoplast membrane** is also a significant cross-reactive antigen with the sarcolemma of the heart. * **Key Concept:** The hyaluronic acid capsule is the reason why *S. pyogenes* is often described as "a wolf in sheep's clothing."

Q: Which marker is lineage-specific for B cells?

CD 19. **Explanation:** The correct answer is **CD 19**. **Why CD 19 is the correct answer:** CD 19 is considered the most reliable **lineage-specific marker** for B cells because it is expressed from the earliest stages of B-cell commitment (pro-B cell) through all stages of maturation, until it is finally downregulated during terminal differentiation into plasma cells. Because it appears before other markers and is present on almost all B-lineage cells, it is the gold standard for identifying B cells in flow cytometry. **Analysis of Incorrect Options:** * **CD 20:** While highly specific to B cells, it is not expressed on the earliest pro-B cells. It is primarily found on mature B cells and is lost during the transition to plasma cells. (Clinical Note: It is the target for **Rituximab**). * **CD 21:** Also known as Complement Receptor 2 (CR2), it is the receptor for the **Epstein-Barr Virus (EBV)**. It is expressed only on mature B cells and follicular dendritic cells, making it less "lineage-encompassing" than CD 19. * **CD 22:** This is a regulatory molecule appearing later in B-cell maturation (mature B cells) and is not present on the earliest precursors. **High-Yield NEET-PG Pearls:** * **Pan B-cell markers:** CD 19, CD 20, and CD 22. * **Plasma Cell Markers:** CD 138 (Syndecan-1) and CD 38. Note that plasma cells typically **lose** CD 19 and CD 20. * **T-cell Lineage Marker:** CD 3 is the definitive lineage-specific marker for T cells. * **NK Cell Markers:** CD 16 and CD 56. * **Hairy Cell Leukemia:** Characterized by CD 11c, CD 25, and CD 103.

Q: Which skin test is based on a neutralization reaction?

Schick test. ### Explanation The **Schick test** is a classic example of an **in vivo neutralization reaction** used to determine immunity against *Corynebacterium diphtheriae*. **1. Why Schick Test is Correct:** The test involves intradermal injection of a sub-lethal dose of **Diphtheria toxin**. * **If the person is immune:** Pre-existing circulating antitoxins (antibodies) neutralize the toxin, resulting in **no reaction** (Negative Schick test). * **If the person is susceptible:** The toxin is not neutralized, causing local inflammation and necrosis (Positive Schick test). Thus, the reaction directly measures the presence of neutralizing antibodies. **2. Why Other Options are Incorrect:** * **Casoni test (Option A):** An immediate **Type I Hypersensitivity** skin test used for diagnosing Hydatid disease (*Echinococcus granulosus*). It relies on IgE-mediated mast cell degranulation. * **Lepromin test (Option B):** A **Type IV (Delayed) Hypersensitivity** reaction used to classify Leprosy and assess the patient's cell-mediated immunity (CMI), not for diagnosis. * **Tuberculin test (Option C):** Also known as the Mantoux test, it is a classic **Type IV Hypersensitivity** reaction. It indicates prior exposure to *M. tuberculosis* and relies on sensitized T-lymphocytes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dick Test:** Another neutralization-based skin test used to identify susceptibility to Scarlet Fever (Streptococcal erythrogenic toxin). * **Schultz-Charlton Reaction:** A neutralization test where antitoxin is injected into a rash to see if it blanches (diagnostic for Scarlet Fever). * **Type IV Hypersensitivity Examples:** Tuberculin, Lepromin, Histoplasmin, and Frei tests. * **Control:** In the Schick test, the opposite arm is injected with **heat-inactivated toxin** to rule out pseudo-reactions (hypersensitivity to bacterial proteins).

Q: Which immunoglobulin activates complement via the alternative pathway?

IgA. **Explanation:** The activation of the complement system occurs via three main pathways: Classical, Alternative, and Lectin. **Why IgA is correct:** While the Classical pathway is triggered by antigen-antibody complexes involving IgG or IgM, the **Alternative pathway** is primarily antibody-independent (triggered by microbial surfaces, endotoxins, or cobra venom). However, among the immunoglobulins, **aggregated IgA** (specifically IgA1 and IgA2) is unique because it can trigger the alternative pathway. It bypasses C1, C4, and C2, acting directly on C3. **Analysis of Incorrect Options:** * **IgM:** This is the most potent activator of the **Classical pathway** due to its pentameric structure, which provides multiple binding sites for the C1q component. * **IgG:** This activates the **Classical pathway**. Among its subclasses, IgG3 is the most effective, followed by IgG1 and IgG2 (IgG4 does not activate complement). * **IgE:** This immunoglobulin is primarily involved in Type I hypersensitivity reactions and helminthic immunity; it does not play a significant role in complement activation. **High-Yield Clinical Pearls for NEET-PG:** * **Classical Pathway:** Triggered by **IgM** (most potent) and **IgG** (1, 2, and 3). Remember: "GM makes it Classic." * **Alternative Pathway:** Triggered by **IgA**, IgD (rarely), endotoxins, and the "Factor" proteins (B, D, and Properdin). * **C3:** This is the common point where all three pathways converge. * **IgA Deficiency:** The most common primary immunodeficiency; patients may have anaphylactic reactions when receiving blood transfusions containing IgA.

Q: A patient presents with cerebellar problems and spider angiomas, diagnosed with ataxia-telangiectasia, a combined T-cell and B-cell deficiency. In addition to a defect in DNA repair enzymes, which immunoglobulin is the primary antibody found in saliva, tears, intestinal, and genital secretions and is also deficient in this condition?

IgA. ### Explanation **Correct Option: B. IgA** **Concept:** Ataxia-telangiectasia (AT) is an autosomal recessive multisystem disorder caused by a mutation in the **ATM gene**, which is responsible for repairing double-stranded DNA breaks. This defect leads to genomic instability, affecting lymphocyte development. While AT is a combined immunodeficiency, the most characteristic and common humoral defect is a **selective deficiency of IgA**. IgA is the primary mediator of **mucosal immunity**. It exists as a monomer in the serum but forms a **dimer** (connected by a J-chain) in secretions. It is the predominant antibody found in "external" fluids such as **saliva, tears, colostrum, bronchial, intestinal, and genitourinary secretions**, where it prevents the attachment of pathogens to epithelial surfaces. **Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** and the only one that crosses the placenta. While IgG subclasses (like IgG2) can be low in AT, it is not the primary secretory antibody. * **IgM:** This is the first antibody produced in a primary immune response and exists as a **pentamer**. It is primarily intravascular and not the dominant secretory immunoglobulin. * **IgD:** Found mainly on the surface of B-cells as an antigen receptor; its systemic secretory function is negligible. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad of AT:** Cerebellar ataxia (staggering gait), Telangiectasia (spider angiomas, especially in the conjunctiva), and Recurrent sinopulmonary infections. * **Laboratory Markers:** Elevated **Alpha-fetoprotein (AFP)** levels (after age 1) are a highly specific diagnostic clue for AT. * **Risk:** Patients have a significantly increased risk of malignancies, particularly **Lymphomas and Leukemias**, due to DNA repair failure. * **Radiology:** Look for **Cerebellar atrophy** on MRI.

Q: Which statement is true about IgM?

It activates complement. **Explanation:** **IgM (Immunoglobulin M)** is the largest antibody (a pentamer) and plays a critical role in the primary immune response. **Why Option B is Correct:** IgM is the **most potent activator of the classical complement pathway**. Due to its pentameric structure, a single molecule of IgM can bind to an antigen and provide multiple Fc binding sites for the C1q component of the complement cascade. In contrast, it takes at least two molecules of IgG to achieve the same effect. **Analysis of Incorrect Options:** * **Option A:** IgM is a **pentamer** in its secreted form (connected by a J-chain), not a dimer. IgA is the primary immunoglobulin that exists as a dimer (specifically in secretions). * **Option C:** IgM **is synthesized by the fetus** starting around 20 weeks of gestation. Because it does not cross the placenta, the detection of IgM in a newborn is a diagnostic marker for **congenital infections** (e.g., TORCH). * **Option D:** IgM **cannot cross the placenta** due to its high molecular weight (approx. 900,000 Daltons), often referred to as the "Millionaire Molecule." **IgG** is the only immunoglobulin class that crosses the placenta. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Pentamer (secreted), Monomer (B-cell surface receptor). * **Primary Response:** It is the first antibody to appear following initial exposure to an antigen. * **Agglutination:** Due to its 10 theoretical binding sites (valency), it is highly efficient at agglutination and cytolysis. * **Isohemagglutinins:** Naturally occurring antibodies against ABO blood groups (Anti-A, Anti-B) are of the IgM class.

Q: Opsonization takes place through which complement component?

C3b. **Explanation:** **Opsonization** is the process by which pathogens are coated with specific molecules (opsonins) to make them more "palatable" and easily recognized by phagocytes (neutrophils and macrophages). **Why C3b is the correct answer:** C3b is the primary opsonin of the complement system. When the complement cascade is activated, C3 is cleaved into C3a and C3b. The **C3b** fragment binds covalently to the surface of bacteria. Phagocytic cells possess specific **CR1 receptors** that bind to C3b, facilitating stable attachment and subsequent engulfment of the pathogen. **Analysis of incorrect options:** * **C3a and C5a:** These are known as **Anaphylatoxins**. They trigger mast cell degranulation, leading to histamine release, increased vascular permeability, and smooth muscle contraction. **C5a** is also a potent chemoattractant for neutrophils. * **C5b:** This fragment serves as the anchor for the formation of the **Membrane Attack Complex (MAC)**. It initiates the terminal pathway by recruiting C6, C7, C8, and C9 to cause osmotic lysis of the target cell. **High-Yield Clinical Pearls for NEET-PG:** * **Major Opsonins:** The two most important opsonins in the body are **C3b** (complement) and **IgG** (specifically the Fc portion). * **C3 Deficiency:** This is the most severe complement deficiency because C3 is the "convergent point" for all three pathways (Classical, Alternative, and Lectin). Deficiency leads to recurrent infections with encapsulated bacteria (e.g., *S. pneumoniae*, *H. influenzae*). * **Alternative Pathway:** C3b is also a key component of the C3 convertase (C3bBb) in the alternative pathway, creating a positive feedback amplification loop.

Q: The Rose-Waaler test is a type of:

Passive agglutination test. ### Explanation The **Rose-Waaler test** is a classic diagnostic method used to detect **Rheumatoid Factor (RF)** in a patient's serum. **1. Why Passive Agglutination is correct:** Agglutination occurs when an antigen and antibody react to form visible clumps. In **Passive Agglutination**, soluble antigens are coated onto the surface of a carrier particle (like RBCs or latex beads) to convert a precipitation reaction into a more visible agglutination reaction. * In the Rose-Waaler test, the carrier particles are **Sheep Red Blood Cells (SRBCs)**. * These SRBCs are "sensitized" by coating them with a sub-agglutinating dose of **rabbit anti-sheep erythrocyte antibody (IgG)**. * If the patient's serum contains Rheumatoid Factor (which is an autoantibody, usually IgM, against the Fc portion of IgG), it reacts with the IgG coating the sheep RBCs, causing visible clumping (agglutination). **2. Why other options are incorrect:** * **Precipitation test:** These involve soluble antigens and antibodies forming an insoluble precipitate. They are less sensitive than agglutination for detecting RF. * **Neutralisation test:** These are used to render toxins or viruses non-infective (e.g., ASO test for Streptolysin O). * **Complement fixation test (CFT):** This involves the consumption of complement during an Ag-Ab reaction, used for infections like Syphilis (Wassermann test), not typically for RF. **3. Clinical Pearls for NEET-PG:** * **Rheumatoid Factor:** Most commonly an **IgM** antibody directed against the **Fc fragment of IgG**. * **Latex Agglutination Test:** A more modern, sensitive version of the Rose-Waaler test using polystyrene latex particles coated with human IgG. * **Sensitivity vs. Specificity:** While Rose-Waaler is more specific for Rheumatoid Arthritis, the Latex test is more commonly used due to its higher sensitivity and ease of use.

Q: Which of the following is an example of a precipitation reaction?

Counter Current Immunoelectrophoresis. **Explanation:** The fundamental principle of a **precipitation reaction** is the interaction between a **soluble antigen** and a **soluble antibody** to form an insoluble visible complex (lattice) in the presence of electrolytes at an optimal temperature and pH. **Why Option C is correct:** **Counter Current Immunoelectrophoresis (CIEP)** is a rapid form of precipitation in gel. It utilizes an electric current to drive the antigen and antibody toward each other. The antigen (usually negatively charged) moves toward the anode, while the antibody moves toward the cathode due to electroendosmosis. When they meet in optimal proportions, a visible **precipitation line** forms. It is significantly faster and more sensitive than standard radial immunodiffusion. **Why other options are incorrect:** * **Widal Test (A) and Weil-Felix Test (D):** These are examples of **Direct Agglutination** reactions. In agglutination, the antigen is **particulate/insoluble** (e.g., whole bacterial cells like *Salmonella* or *Proteus*). When they react with antibodies, they form visible clumps rather than precipitates. * **Coombs’ Test (B):** This is an **Antiglobulin Test** (Indirect Agglutination). It uses antihuman globulin to detect "incomplete" antibodies that are already coated on the surface of Red Blood Cells but are unable to cause visible agglutination on their own. **NEET-PG High-Yield Pearls:** * **Precipitation** = Soluble Ag + Soluble Ab. * **Agglutination** = Particulate Ag + Soluble Ab. * **VDRL & Kahn Test:** Classic examples of **Flocculation** (a subtype of precipitation where the precipitate remains suspended as flakes). * **Elek’s Test:** A specialized precipitation test (toxigenicity test) used for *Corynebacterium diphtheriae*. * **Zone Phenomenon:** Precipitation is most visible in the **Zone of Equivalence**. Excess of either antibody (Prozone) or antigen (Postzone) can lead to false-negative results.

Q: Which of the following immunoglobulins acts as a receptor for antigens?

IgD. ### Explanation **Correct Option: C (IgD)** The primary function of **IgD** is to act as an **antigen receptor** on the surface of mature, naive B-lymphocytes. Along with monomeric IgM, IgD is co-expressed on the B-cell membrane before the cell has encountered its specific antigen. When an antigen binds to these surface receptors, it triggers B-cell activation, leading to clonal expansion and differentiation into plasma cells. Unlike other immunoglobulins, IgD is secreted in very trace amounts in the serum and has no well-defined effector function (like opsonization or complement activation) once secreted. **Incorrect Options:** * **IgG:** This is the most abundant serum immunoglobulin. Its primary roles are opsonization, complement activation, and providing passive immunity to the fetus (crosses the placenta). It does not serve as a primary B-cell receptor. * **IgM:** While the **monomeric** form of IgM acts as a B-cell receptor alongside IgD, the **pentameric** form is the first antibody produced in a primary immune response and is highly effective at complement fixation. * **IgE:** This immunoglobulin is primarily involved in Type I hypersensitivity reactions (allergy) and provides immunity against helminthic parasites by binding to Fc receptors on mast cells and basophils. **High-Yield Clinical Pearls for NEET-PG:** * **B-cell Markers:** Mature naive B-cells are characterized by the presence of both **surface IgM and IgD**. * **Heat Lability:** IgD and IgE are the most heat-labile immunoglobulins. * **Hinge Region:** IgD has the longest hinge region, making it highly susceptible to proteolytic degradation. * **Memory B-cells:** Once a B-cell undergoes class switching (e.g., to IgG), it loses the expression of IgD.

Question 1

Which of the following streptococcal antigens cross-reacts with synovial fluid?

Accepted Answer

Capsular hyaluronic acid

Answer

Carbohydrate (group A)

Answer

Cell wall protein

Answer

Peptidoglycan

Question 2

Which marker is lineage-specific for B cells?

Accepted Answer

CD 19

Answer

CD 20

Answer

CD 21

Answer

CD 22

Question 3

Which skin test is based on a neutralization reaction?

Accepted Answer

Schick test

Answer

Casoni test

Answer

Lepromin test

Answer

Tuberculin test

Question 4

Which immunoglobulin activates complement via the alternative pathway?

Accepted Answer

IgA

Answer

IgE

Answer

IgM

Answer

IgG

Question 5

A patient presents with cerebellar problems and spider angiomas, diagnosed with ataxia-telangiectasia, a combined T-cell and B-cell deficiency. In addition to a defect in DNA repair enzymes, which immunoglobulin is the primary antibody found in saliva, tears, intestinal, and genital secretions and is also deficient in this condition?

Accepted Answer

IgA

Answer

IgG

Answer

IgM

Answer

IgD

Question 6

Which statement is true about IgM?

Accepted Answer

It activates complement

Answer

It is dimeric

Answer

It is not synthesized by the fetus

Answer

It is the last immunoglobulin to cross the placenta

Question 7

Opsonization takes place through which complement component?

Accepted Answer

C3b

Answer

C3a

Answer

C5a

Answer

C5b

Question 8

The Rose-Waaler test is a type of:

Accepted Answer

Passive agglutination test

Answer

Precipitation test

Answer

Neutralisation test

Answer

Complement fixation test

Question 9

Which of the following is an example of a precipitation reaction?

Accepted Answer

Counter Current Immunoelectrophoresis

Answer

Widal Test

Answer

Coomb's Test

Answer

Weil-Felix Test

Question 10

Which of the following immunoglobulins acts as a receptor for antigens?

Accepted Answer

IgD

Answer

IgG

Answer

IgM

Answer

IgE

Immunology — MCQs

Immunology — MCQs

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