Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 671: Large granular lymphocytes are a subset of which cell type?

A. Neutrophils
B. Macrophages
C. Eosinophils
D. Lymphocytes (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Lymphocytes** **Concept Overview:** Large Granular Lymphocytes (LGLs) represent a morphologically distinct subset of lymphocytes characterized by a larger size than resting T or B cells, an eccentric nucleus, and the presence of prominent azurophilic granules in the cytoplasm. These granules contain cytotoxic proteins like **perforins and granzymes**. The term LGL primarily refers to two functional groups: 1. **Natural Killer (NK) Cells:** These make up the majority (~85%) of LGLs. They are part of the innate immune system and do not require prior sensitization to kill virus-infected or tumor cells. 2. **Activated CD8+ T-cells (Cytotoxic T-lymphocytes):** A smaller subset of LGLs that are part of the adaptive immune system. --- **Why Incorrect Options are Wrong:** * **A. Neutrophils:** These are granulocytes belonging to the myeloid lineage. While they contain granules, they possess multi-lobed nuclei (polymorphonuclear) and are distinct from the lymphoid lineage. * **B. Macrophages:** These are large mononuclear phagocytes derived from monocytes. They have a "frothy" or vacuolated cytoplasm rather than the specific azurophilic granulation seen in LGLs. * **C. Eosinophils:** These are myeloid cells characterized by coarse, brick-red (eosinophilic) granules and typically a bilobed nucleus. --- **NEET-PG High-Yield Pearls:** * **NK Cell Markers:** CD16 (FcγRIII) and CD56 are the characteristic surface markers for NK cells. * **Mechanism of Action:** NK cells utilize the **"Missing Self" hypothesis**, where they kill cells that lack or have downregulated MHC Class I molecules (a common viral/tumor escape mechanism). * **Clinical Correlation:** **LGL Leukemia** is a rare lymphoproliferative disorder characterized by a persistent increase in these cells, often associated with rheumatoid arthritis and neutropenia (similar to Felty’s syndrome).

Question 672: C3 convertase acts on which of the following substrates?

A. C4b2b
B. C4b2B3b
C. C4b
D. C3 (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. C3** **Concept:** The complement system is a cascade of proteins that mediate inflammation and pathogen clearance. The central event in all three pathways (Classical, Alternative, and Lectin) is the formation of **C3 convertase**. As the name implies, a "convertase" is an enzyme that acts upon its specific substrate. C3 convertase cleaves **C3** into C3a (anaphylatoxin) and C3b (opsonin). This step is the most critical amplification point in the complement cascade. **Analysis of Options:** * **Option A (C4b2b):** This is the **Classical/Lectin pathway C3 convertase** itself. It is the enzyme, not the substrate. It acts *on* C3. * **Option B (C4b2b3b):** This is the **Classical pathway C5 convertase**. It is formed when C3b attaches to the C3 convertase. Its substrate is C5, not C3. * **Option C (C4b):** This is a fragment of C4 that acts as a structural component/scaffold for the formation of the C3 convertase (C4b2b), but it is not the substrate being cleaved by the convertase enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Alternative Pathway C3 Convertase:** Unlike the classical pathway (C4b2b), the alternative pathway C3 convertase is **C3bBb**. * **C3 Deficiency:** This is the most severe complement deficiency because it compromises all three pathways, leading to recurrent pyogenic infections (e.g., *S. pneumoniae*, *H. influenzae*). * **Anaphylatoxins:** C3a, C4a, and C5a (in increasing order of potency: C5a > C3a > C4a) trigger mast cell degranulation. * **Opsonization:** C3b is the primary opsonin that enhances phagocytosis by binding to CR1 receptors on macrophages.

Question 673: What is the half-life of IgG?

A. 5 days
B. 6 days
C. 8 days
D. 23 days (Correct Answer)

Explanation: **Explanation:** The correct answer is **23 days (Option D)**. **Understanding the Concept:** Immunoglobulin G (IgG) is the most abundant antibody in the serum, accounting for approximately 75-80% of the total pool. It has the **longest half-life** of all immunoglobulins, averaging **23 days** (specifically for subclasses IgG1, IgG2, and IgG4; IgG3 is an exception with a half-life of about 7 days). This prolonged half-life is due to the **neonatal Fc receptor (FcRn)**. This receptor binds to IgG in endosomes, protecting it from lysosomal degradation and recycling it back into the circulation. **Analysis of Incorrect Options:** * **A (5 days):** This is the approximate half-life of **IgA**. * **B (6 days):** This is the approximate half-life of **IgM**. * **C (8 days):** This is the approximate half-life of **IgD** (approx. 3 days) or **IgG3** (approx. 7-9 days), but it does not represent the majority of IgG. **High-Yield Clinical Pearls for NEET-PG:** * **Placental Transfer:** IgG is the **only** immunoglobulin that crosses the placenta, providing passive immunity to the newborn. * **Secondary Response:** IgG is the predominant antibody produced during the **anamnestic (secondary) immune response**. * **Subclasses:** IgG has four subclasses. **IgG1** is the most abundant; **IgG2** is vital for defense against capsulated bacteria; **IgG3** is the most effective complement fixer. * **Agglutination:** IgG is a "late" antibody and is considered a **monomer** (valence of 2). Unlike IgM, it is a poor agglutinator but an excellent opsonin.

Question 674: Which one of the following microorganisms uses antigenic variation as a major means of evading host defenses?

A. Streptococcus pneumoniae
B. Borrelia recurrentis (Correct Answer)
C. Mycobacterium tuberculosis
D. Listeria monocytogenes

Explanation: **Explanation:** **Correct Answer: B. Borrelia recurrentis** *Borrelia recurrentis*, the causative agent of louse-borne relapsing fever, is the classic example of a pathogen that utilizes **programmed antigenic variation** to evade the host immune system. The bacteria possess a large repertoire of genes encoding **Variable Large Proteins (VLP)** on their outer membrane. Through gene conversion and rearrangement, the bacteria periodically switch these surface proteins. * **Mechanism:** When the host produces antibodies against the dominant serotype, the bacteria are cleared from the blood (afebrile period). However, a small subpopulation switches to a new antigenic variant that the existing antibodies cannot recognize. These multiply, leading to a new wave of bacteremia and fever (the "relapse"). **Why other options are incorrect:** * **A. Streptococcus pneumoniae:** Evades the immune system primarily through its **polysaccharide capsule**, which prevents phagocytosis. While there are many serotypes, an individual strain does not switch its antigens during a single infection. * **C. Mycobacterium tuberculosis:** Uses **intracellular survival** within macrophages by inhibiting phagosome-lysosome fusion. * **D. Listeria monocytogenes:** Escapes host defenses by using **Listeriolysin O** to break out of the phagosome and moving cell-to-cell via **actin polymerization** (actin rockets). **NEET-PG High-Yield Pearls:** * **Other organisms using antigenic variation:** *Neisseria gonorrhoeae* (pili), *Trypanosoma brucei* (VSG genes), and Influenza virus (antigenic drift/shift). * **Relapsing Fever:** Characterized by 3–10 relapses; diagnosis is made by seeing spirochetes on a **peripheral blood smear** (Giemsa/Wright stain) during the febrile phase. * **Jarisch-Herxheimer reaction:** A common systemic inflammatory response seen shortly after starting antibiotics for *Borrelia* infections.

Question 675: Which portion of an antibody molecule binds to an antigen?

A. Hinge region
B. Constant region
C. Variable region
D. Hypervariable region (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Hypervariable region.** **Understanding the Concept:** An antibody (Immunoglobulin) is a Y-shaped molecule consisting of two heavy (H) and two light (L) chains. Both chains have **Variable (V)** and **Constant (C)** domains. Within the variable domains of both H and L chains, there are specific sub-regions where amino acid sequences vary most significantly; these are called **Hypervariable regions** or **Complementarity Determining Regions (CDRs)**. While the "Variable region" (Option C) as a whole is involved in antigen recognition, the **Hypervariable regions** are the specific, precise sites that form the antigen-binding pocket (paratope). They provide the structural complementarity required to bind to a specific epitope on an antigen. **Why other options are incorrect:** * **Hinge region:** This is a flexible amino acid stretch between the CH1 and CH2 domains. It allows the two antigen-binding arms to move independently, but it does not bind to antigens. * **Constant region:** This part of the antibody (Fc portion) determines the biological properties of the immunoglobulin (e.g., placental transfer, complement fixation, binding to mast cells) and is identical for all antibodies of the same isotype. * **Variable region:** This is a broader term. While it contains the binding site, the **Hypervariable region** is the more specific and accurate answer for the actual binding interface. **High-Yield Clinical Pearls for NEET-PG:** * **Paratope:** The part of the antibody that binds to the antigen (formed by hypervariable regions). * **Epitope:** The specific part of the antigen that binds to the antibody. * **Papain Digestion:** Cleaves the antibody into **two Fab fragments** (antigen-binding) and **one Fc fragment** (crystallizable/constant). * **Pepsin Digestion:** Cleaves the antibody into **one F(ab')2 fragment** and degraded Fc fragments. * **Idiotype:** Determined by the hypervariable regions; it is unique to each individual antibody molecule.

Question 676: Which CD marker is characteristically associated with B lymphocytes?

A. CD 19 (Correct Answer)
B. CD 27
C. CD 4
D. CD 35

Explanation: ### Explanation **Correct Option: A (CD 19)** CD 19 is considered the most reliable and characteristic marker for **B lymphocytes**. It is expressed on the surface of B cells from the earliest stages of pre-B cell development until the stage just before terminal differentiation into plasma cells. It functions as a co-receptor with CD21 and CD81 to lower the threshold for B-cell antigen receptor signaling. In clinical practice, CD19 is the primary marker used for B-cell quantification in flow cytometry. **Analysis of Incorrect Options:** * **CD 27:** While CD27 is found on B cells, it is specifically a marker for **Memory B cells**. It is not universal to all B cell stages, making CD19 a more characteristic general marker. * **CD 4:** This is a characteristic marker for **Helper T lymphocytes** and is also expressed on monocytes and macrophages. It serves as the primary receptor for HIV entry. * **CD 35:** Also known as Complement Receptor 1 (CR1), it is found on various cells including erythrocytes, neutrophils, and B cells. It is not specific to B lymphocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Pan-B cell markers:** CD19, CD20, and CD22. * **Plasma Cell Marker:** CD138 (Note: Mature plasma cells usually lose CD19 and CD20). * **Pan-T cell marker:** CD3 (associated with the T-cell receptor). * **NK cell markers:** CD16 (FcγRIII) and CD56. * **Rituximab:** A monoclonal antibody used in lymphomas and autoimmune diseases that targets **CD20**. * **Epstein-Barr Virus (EBV) Receptor:** CD21 (CR2) on B cells is the attachment site for EBV.

Question 677: What type of receptors are present on T cells?

A. IgG
B. IgD
C. CD4 (Correct Answer)
D. Prostaglandins

Explanation: **Explanation:** The correct answer is **CD4**. T cells are characterized by the presence of specific surface markers known as **Cluster of Differentiation (CD)** molecules. These receptors are essential for antigen recognition and immune signaling. * **Why CD4 is correct:** T cells are broadly divided into two functional subsets based on their surface receptors: **Helper T cells (CD4+)** and **Cytotoxic T cells (CD8+)**. CD4 acts as a co-receptor that binds to the non-polymorphic region of **MHC Class II** molecules on Antigen-Presenting Cells (APCs), facilitating the activation of the T-cell receptor (TCR) complex. **Analysis of Incorrect Options:** * **A & B (IgG and IgD):** These are types of Immunoglobulins (antibodies). While **IgD and IgM** serve as the primary B-cell receptors (BCRs), they are not found on T cells. T cells use a T-cell receptor (TCR) instead of membrane-bound antibodies to recognize antigens. * **D (Prostaglandins):** These are lipid-derived inflammatory mediators, not structural receptors found on the surface of T cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **MHC Restriction:** Remember the **"Rule of 8"**: CD4 cells recognize MHC II (4 × 2 = 8), and CD8 cells recognize MHC I (8 × 1 = 8). 2. **Pan-T cell marker:** **CD3** is the definitive marker present on *all* mature T cells and is associated with the TCR. 3. **HIV Pathogenesis:** The HIV virus specifically targets the **CD4 receptor** (using its gp120 envelope protein) to enter and destroy Helper T cells, leading to immunodeficiency. 4. **Th1 vs Th2:** CD4+ cells further differentiate into Th1 (cell-mediated immunity) and Th2 (humoral immunity) based on the cytokine environment.

Question 678: Which of the following does not secrete interleukin-1 alpha?

A. Lymphocyte
B. Fibroblast
C. Macrophage
D. Neutrophil (Correct Answer)

Explanation: **Explanation:** Interleukin-1 (IL-1) is a key pro-inflammatory cytokine that exists in two primary forms: **IL-1α** and **IL-1β**. While they share the same receptor, their cellular distribution and release mechanisms differ. **Why Neutrophil is the correct answer:** Neutrophils are primarily producers of **IL-1β**, not IL-1α. IL-1α is typically constitutively expressed in epithelial and mesenchymal cells or produced by specific immune cells like macrophages and lymphocytes. Neutrophils focus on the rapid release of IL-1β via the inflammasome pathway during acute inflammation. Therefore, they are generally considered non-secretors of IL-1α. **Analysis of Incorrect Options:** * **Macrophage:** These are the primary professional sources of both IL-1α and IL-1β. They produce IL-1α as a membrane-bound form and a secreted cytokine to initiate the inflammatory cascade. * **Lymphocyte:** Both B-cells and certain T-cell subsets can produce IL-1α, particularly during activation and interaction with antigen-presenting cells. * **Fibroblast:** These are non-immune cells (mesenchymal) that produce IL-1α, especially in response to local tissue injury, acting as an "alarmin" to signal damage to the immune system. **High-Yield NEET-PG Pearls:** * **IL-1α vs. IL-1β:** IL-1α is often cell-associated (membrane-bound) and acts as an **alarmin** (released upon cell death), whereas IL-1β is strictly secreted and requires cleavage by **Caspase-1** (via the inflammasome). * **Endogenous Pyrogen:** IL-1 is a potent endogenous pyrogen that acts on the hypothalamus to induce fever. * **Clinical Correlation:** **Anakinra** is a recombinant IL-1 receptor antagonist used in the treatment of Rheumatoid Arthritis and Autoinflammatory syndromes.

Question 679: Which of the following is NOT a pyrogen?

A. Interferon-alpha (IFN-α)
B. Tumor Necrosis Factor-alpha (TNF-α)
C. Interleukin-4 (IL-4)
D. Interleukin-18 (IL-18) (Correct Answer)

Explanation: **Explanation:** Pyrogens are substances that induce fever by acting on the hypothalamus to increase the thermoregulatory set-point. They are classified into **Exogenous pyrogens** (e.g., LPS/Endotoxin) and **Endogenous pyrogens** (pro-inflammatory cytokines). **Why Interleukin-18 (IL-18) is the correct answer:** While IL-18 belongs to the IL-1 family (similar to IL-1β), its primary role is inducing IFN-γ production and enhancing NK cell activity. Unlike its counterparts, IL-18 does not possess significant pyrogenic properties and is not typically involved in the systemic febrile response. **Analysis of Incorrect Options:** * **Interleukin-1 (IL-1):** Often considered the "master pyrogen," it is the most potent inducer of fever. (Note: IL-18 is the outlier in this family). * **Tumor Necrosis Factor-alpha (TNF-α):** A major endogenous pyrogen that induces fever both directly and by stimulating the release of IL-1. * **Interferon-alpha (IFN-α):** Known to produce "flu-like symptoms," including significant fever, often seen as a side effect during clinical administration for hepatitis or malignancies. * **Interleukin-6 (IL-6):** (Often tested) A key mediator that crosses the blood-brain barrier to stimulate prostaglandin E2 (PGE2) production in the hypothalamus. **High-Yield Clinical Pearls for NEET-PG:** * **The Final Mediator:** PGE2 is the ultimate mediator of fever in the hypothalamus. Aspirin and NSAIDs work by inhibiting Cyclooxygenase (COX), thereby blocking PGE2 synthesis. * **Potency Hierarchy:** IL-1 > TNF-α > IL-6. * **Exogenous Pyrogen:** The most common is the Lipopolysaccharide (LPS) of Gram-negative bacteria (Endotoxin).

Question 680: Prausnitz-kustner reaction is a

A. Type I Hypersensitivity (Correct Answer)
B. Type II Hypersensitivity
C. Type III Hypersensitivity
D. Type IV Hypersensitivity

Explanation: **Explanation:** The **Prausnitz-Küstner (PK) reaction** is a classic historical demonstration of **Type I Hypersensitivity** (Immediate Hypersensitivity). It involves the passive transfer of serum from an allergic individual (containing IgE antibodies, formerly called "reagins") into the skin of a non-allergic recipient. When the specific allergen is later injected into the same site, a wheal-and-flare reaction occurs within minutes, proving that the allergic mediator is present in the serum. **Why the other options are incorrect:** * **Type II (Cytotoxic):** Involves IgG or IgM antibodies binding to antigens on cell surfaces (e.g., ABO incompatibility, Myasthenia Gravis). It does not involve IgE-mediated mast cell degranulation. * **Type III (Immune-complex):** Mediated by antigen-antibody complexes depositing in tissues (e.g., SLE, Arthus reaction). These reactions typically take 3–10 hours to manifest. * **Type IV (Delayed-type):** Cell-mediated immunity involving T-lymphocytes rather than antibodies (e.g., Mantoux test, Contact dermatitis). These reactions peak at 48–72 hours. **High-Yield Clinical Pearls for NEET-PG:** * **Mediator:** The PK reaction specifically identifies **IgE** (the only heat-labile antibody). * **Mechanism:** IgE binds to **FcεRI receptors** on mast cells in the recipient's skin. * **Safety Note:** This test is no longer used clinically due to the risk of transmitting blood-borne pathogens (like HIV or Hepatitis B/C). * **Casoni’s Test:** Another example of a Type I Hypersensitivity skin test used for Hydatid disease.

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Immunology — MCQs

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