Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 881: Which statement is true about IgM?

A. It is dimeric
B. It activates complement (Correct Answer)
C. It is not synthesized by the fetus
D. It is the last immunoglobulin to cross the placenta

Explanation: **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.

Question 882: Berson and Yalow first described which of the following tests?

A. Radioimmunoassay (RIA) (Correct Answer)
B. Enzyme-Linked Immunosorbent Assay (ELISA)
C. Immunochromatography
D. Chemiluminescence assay

Explanation: **Explanation:** **Correct Answer: A. Radioimmunoassay (RIA)** Solomon Berson and Rosalyn Yalow first described the **Radioimmunoassay (RIA)** in 1960. They initially developed this technique to measure endogenous plasma insulin levels. The discovery was revolutionary because it allowed for the detection of extremely minute concentrations of antigens or antibodies using radioactive isotopes (typically Iodine-125). Rosalyn Yalow was awarded the **Nobel Prize in Medicine (1977)** for this achievement. **Analysis of Incorrect Options:** * **B. ELISA:** This technique was developed later (1971) by Peter Perlmann and Eva Engvall (and independently by Schuurs and Weemen) as a safer alternative to RIA, replacing radioactive isotopes with enzyme markers. * **C. Immunochromatography:** This is the principle behind "Lateral Flow Assays" (e.g., rapid pregnancy tests or COVID-19 RAT). It was developed in the 1980s and relies on the migration of antigen-antibody complexes across a membrane. * **D. Chemiluminescence assay (CLIA):** This is a modern, highly sensitive technique that uses chemical reactions to produce light (luminescence) rather than radioactivity or color changes. **High-Yield Clinical Pearls for NEET-PG:** * **Principle of RIA:** It is based on **competitive binding**. A fixed amount of labeled (radioactive) antigen competes with an unknown amount of unlabeled (patient) antigen for a limited number of antibody binding sites. * **Sensitivity:** RIA can detect concentrations as low as **nanograms or picograms**, making it useful for measuring hormones (TSH, Insulin), drugs (Digoxin), and vitamins (B12). * **Prozone Phenomenon:** Remember that RIA and ELISA are less prone to the prozone phenomenon compared to traditional precipitation/agglutination tests.

Question 883: Which one of the following immunoglobulins constitutes the antigen binding component of the B-cell receptor?

A. IgA
B. IgD (Correct Answer)
C. IgM
D. IgG

Explanation: **Explanation:** The **B-cell receptor (BCR)** is a transmembrane protein complex located on the surface of B-cells. It is composed of a membrane-bound immunoglobulin (mIg) molecule and a signal-transducing heterodimer (Igα and Igβ). In **mature, naive B-cells** (B-cells that have left the bone marrow but have not yet encountered an antigen), both **IgD** and **IgM** are co-expressed on the cell surface. While both function as receptors, **IgD** is specifically recognized as a primary antigen-binding component of the BCR on these mature cells. Its presence serves as a marker for B-cell maturity; once a B-cell is activated by an antigen, IgD expression is typically lost. **Analysis of Options:** * **Option B (IgD):** Correct. Along with IgM, IgD serves as the membrane-bound receptor for antigen recognition on mature naive B-lymphocytes. * **Option C (IgM):** While IgM is also part of the BCR, in the context of standardized medical exams like NEET-PG, if both are listed, IgD is often highlighted for its specific role as a surface receptor rather than a secreted antibody. (Note: In many contexts, IgM is also correct, but IgD is the classic "textbook" answer for the BCR component of mature cells). * **Option A (IgA):** Primarily found in secretions (tears, saliva, colostrum) as a dimer; it provides mucosal immunity. * **Option D (IgG):** The most abundant circulating antibody; it crosses the placenta and mediates secondary immune responses but is not the primary BCR on naive cells. **High-Yield Facts for NEET-PG:** * **B-cell Maturity:** Immature B-cells express only surface IgM; **Mature B-cells express both IgM and IgD.** * **IgD Function:** It has no known significant effector function as a secreted antibody; its role is almost exclusively as a BCR. * **Signal Transduction:** The BCR itself has short cytoplasmic tails; signaling is actually carried out by the associated **Igα (CD79a)** and **Igβ (CD79b)** molecules.

Question 884: What are opsonins?

A. C3a (Correct Answer)
B. IgM
C. Carbohydrate binding proteins
D. Selectins

Explanation: **Explanation:** **Opsonization** is the process by which foreign particles (like bacteria) are coated with specific molecules called **opsonins** to make them more "palatable" and easily recognized by phagocytes (neutrophils and macrophages). **Why C3a is the correct answer:** The complement system produces several fragments that act as opsonins. **C3b** is the primary and most potent opsonin. However, in many standardized exams including NEET-PG, **C3a** (and sometimes C4b) are categorized within the opsonization pathway. C3a primarily acts as an anaphylatoxin, but it plays a crucial role in the recruitment and activation of phagocytes, facilitating the overall process of opsonization and clearance. **Analysis of Incorrect Options:** * **B. IgM:** While antibodies are opsonins, **IgG** (specifically IgG1 and IgG3) is the primary opsonizing antibody. IgM is a poor opsonin because phagocytes lack specific receptors for the Fc portion of IgM; it aids phagocytosis indirectly by activating the complement pathway to produce C3b. * **C. Carbohydrate binding proteins:** While some (like Mannose-Binding Lectin) act as opsonins, the term is too broad and non-specific compared to a direct complement component. * **D. Selectins:** These are adhesion molecules (L, E, and P-selectins) involved in the **rolling** phase of leukocyte extravasation, not opsonization. **NEET-PG High-Yield Pearls:** * **Most powerful opsonins:** IgG and C3b ("**G**o **B**efore" to remember IgG and C3b). * **Anaphylatoxins:** C3a, C4a, and C5a (induce mast cell degranulation). * **Chemotaxis:** C5a is the most potent chemoattractant for neutrophils. * **Mechanism:** Opsonins overcome the negative electrostatic charge on bacterial surfaces, allowing the negatively charged phagocyte to bind effectively.

Question 885: Opsonization takes place through which complement component?

A. C3a
B. C3b (Correct Answer)
C. C5a
D. C5b

Explanation: **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.

Question 886: Which of the following staphylococcal proteins functions as a superantigen?

A. Exfoliative toxin (Correct Answer)
B. Cytolytic toxin
C. Protein A
D. Leucocidin

Explanation: ### Explanation **Correct Answer: A. Exfoliative toxin** **Concept:** Superantigens are potent immunostimulatory molecules that bypass normal antigen processing. Instead of being presented in the MHC II groove, they bind directly to the **outer surface of MHC II** on antigen-presenting cells and the **Vβ region of T-cell receptors (TCR)**. This results in a massive, non-specific activation of T-cells (up to 20%), leading to a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ). Staphylococcal superantigens include: 1. **Toxic Shock Syndrome Toxin-1 (TSST-1)** 2. **Staphylococcal Enterotoxins** (A-E, cause food poisoning) 3. **Exfoliative Toxins (ETA, ETB):** These cause Staphylococcal Scalded Skin Syndrome (SSSS) by acting as superantigens and proteases that cleave **desmoglein-1**, leading to epidermal separation. --- ### Why the other options are incorrect: * **B. Cytolytic toxins:** (e.g., Alpha, Beta, Gamma, Delta toxins) These act by forming pores in the host cell membrane or through enzymatic action, leading to cell lysis, rather than T-cell activation. * **C. Protein A:** This is a surface protein that binds to the **Fc portion of IgG**, preventing opsonization and phagocytosis. It is an antiphagocytic virulence factor, not a superantigen. * **D. Leucocidin:** (e.g., Panton-Valentine Leucocidin) This is a pore-forming toxin that specifically targets and kills polymorphonuclear leukocytes (neutrophils) and macrophages. --- ### High-Yield Facts for NEET-PG: * **MHC Binding:** Superantigens bind to the **Vβ chain** of the TCR. * **SSSS vs. TEN:** In SSSS (caused by Exfoliative toxin), the cleavage occurs at the **stratum granulosum**, and the Nikolsky sign is positive. Unlike Toxic Epidermal Necrolysis (TEN), there is typically no mucosal involvement. * **Enterotoxin:** Staphylococcal enterotoxin is **heat-stable** and causes "pre-formed toxin" food poisoning (short incubation: 1–6 hours).

Question 887: Trapping of parasites by Neutrophil Extracellular Traps (NETs) is assisted by which of the following cell types?

A. Neutrophils and Eosinophils (Correct Answer)
B. Neutrophils and Monocytes
C. Eosinophils and Monocytes
D. NK cells

Explanation: ### **Explanation** **1. Why Neutrophils and Eosinophils are Correct:** The concept of **NETosis** refers to a unique form of programmed cell death where cells release a web-like scaffold of chromatin (DNA) decorated with antimicrobial proteins to trap and kill pathogens extracellularly. * **Neutrophils:** These are the primary cells associated with **NETs**. They release DNA fibers embedded with enzymes like myeloperoxidase (MPO) and neutrophil elastase to trap bacteria, fungi, and parasites. * **Eosinophils:** These cells produce **EETs (Eosinophil Extracellular Traps)**. Since eosinophils are the primary defense against helminthic parasites (too large to be phagocytosed), they release mitochondrial or nuclear DNA to immobilize and neutralize larvae and parasites. **2. Analysis of Incorrect Options:** * **Monocytes (Options B & C):** While monocytes can undergo a similar process (METs), their primary role is phagocytosis and cytokine production. In the context of parasite trapping, the synergy between neutrophils and eosinophils is the physiological hallmark. * **NK cells (Option D):** Natural Killer cells function primarily through the release of perforins and granzymes to induce apoptosis in virally infected or tumor cells; they do not form extracellular DNA traps. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **NETosis Mechanism:** It is a **NADPH oxidase-dependent** process. A deficiency in this enzyme (as seen in **Chronic Granulomatous Disease**) leads to impaired NET formation and recurrent infections. * **Components:** NETs consist of a **DNA backbone** + **Histones** + **Granule proteins** (Elastase, MPO, Cathepsin G). * **Double-edged Sword:** While NETs are protective, excessive NET formation is implicated in autoimmune diseases like **SLE** (source of autoantigens) and **thrombosis** (provides a scaffold for platelet aggregation). * **Parasite Defense:** Eosinophils are specifically recruited via **IL-5** and use Major Basic Protein (MBP) alongside EETs to combat parasites.

Question 888: The Rose-Waaler test is a type of:

A. Precipitation test
B. Passive agglutination test (Correct Answer)
C. Neutralisation test
D. Complement fixation test

Explanation: ### 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.

Question 889: CD4 is a marker for which of the following cell types?

A. Stem cells
B. All leukocytes
C. B cells
D. T cells (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. T cells** CD4 (Cluster of Differentiation 4) is a glycoprotein found primarily on the surface of **Helper T cells (Th cells)**. In the immune system, CD4 acts as a co-receptor that assists the T-cell receptor (TCR) in communicating with antigen-presenting cells. Specifically, CD4 recognizes and binds to the **MHC Class II** molecules. This interaction is essential for the activation of the adaptive immune response. **Analysis of Incorrect Options:** * **A. Stem cells:** Hematopoietic stem cells are characterized by markers like **CD34**. They do not express mature lineage markers like CD4. * **B. All leukocytes:** This is incorrect. While all leukocytes express **CD45** (the Leukocyte Common Antigen), CD4 is specific to a subset of lymphocytes and some myeloid cells (like monocytes/macrophages, though at lower levels). * **C. B cells:** B cells are identified by markers such as **CD19, CD20, and CD21**. They do not express CD4. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction Rule:** Remember the **"Rule of 8"**: CD4 cells bind to MHC II (4 × 2 = 8), while CD8 cells bind to MHC I (8 × 1 = 8). * **HIV Pathogenesis:** The HIV virus uses the **CD4 molecule as its primary receptor** (along with co-receptors CCR5 or CXCR4) to enter cells. A CD4 count <200 cells/mm³ is a diagnostic criterion for AIDS. * **CD4:CD8 Ratio:** In a healthy individual, the normal ratio is approximately **2:1**. This ratio is characteristically inverted in HIV/AIDS. * **Other CD4+ cells:** While primarily a T-cell marker, CD4 is also weakly expressed on **monocytes, macrophages, and dendritic cells**.

Question 890: Immunoglobulins are produced by which of the following cells?

A. Macrophages
B. B-cells (Correct Answer)
C. T-cells
D. NK-cells

Explanation: ### Explanation **Correct Option: B (B-cells)** Immunoglobulins (antibodies) are glycoproteins produced by the humoral arm of the adaptive immune system. When a **B-lymphocyte** encounters a specific antigen, it undergoes activation, proliferation, and differentiation into **Plasma cells**. These plasma cells are the "antibody factories" of the body, secreting soluble immunoglobulins (IgG, IgA, IgM, IgE, and IgD) into the blood and lymph to neutralize pathogens. **Why Incorrect Options are Wrong:** * **A. Macrophages:** These are myeloid-derived cells primarily involved in **phagocytosis** and acting as Professional Antigen Presenting Cells (APCs). They do not produce antibodies. * **C. T-cells:** These are responsible for **cell-mediated immunity**. CD4+ T-cells (Helper) coordinate the immune response, while CD8+ T-cells (Cytotoxic) kill virally infected cells directly. They do not secrete immunoglobulins. * **D. NK-cells:** Natural Killer cells are part of the **innate immune system**. They provide a rapid response to virally infected cells and tumor cells through granzymes and perforins, not through antibody production. **High-Yield Clinical Pearls for NEET-PG:** * **Plasma Cell Morphology:** Characterized by an eccentric nucleus with a **"Cart-wheel"** or "Clock-face" chromatin pattern and a prominent perinuclear halo (Golgi apparatus). * **Multiple Myeloma:** A plasma cell dyscrasia (malignancy) leading to the overproduction of monoclonal immunoglobulins (M-spike on electrophoresis). * **B-cell Markers:** CD19, CD20, and CD21 are the characteristic surface markers used to identify B-cells. * **Isotype Switching:** While B-cells initially produce IgM, they can switch to producing IgG, IgA, or IgE under the influence of cytokines from T-helper cells.

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Immunology — MCQs

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