Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 771: On which component of the complement system does C3 convertase act?

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

Explanation: **Explanation:** The complement system is a biochemical cascade of the innate immune system. The central event in all three pathways (Classical, Alternative, and Lectin) is the formation of **C3 convertase**. **Why Option A is correct:** C3 convertase is an enzyme complex whose specific substrate is the **C3 protein**. It cleaves C3 into two fragments: **C3a** (an anaphylatoxin) and **C3b** (an opsonin). This step is the "amplification loop" of the complement cascade, as one molecule of C3 convertase can cleave hundreds of C3 molecules, leading to massive deposition of C3b on the pathogen surface. **Why other options are incorrect:** * **Option B (C4b2b):** This is not a substrate; it is the **Classical/Lectin pathway C3 convertase** itself. It acts *on* C3, it is not acted upon by C3 convertase. * **Option C (C4b):** This is a fragment of C4 that combines with C2b to form the C3 convertase complex. It is a structural component, not the primary substrate of the convertase. * **Option D (Lymphocytosis):** This refers to an increased lymphocyte count in the blood, typically seen in viral infections or chronic lymphocytic leukemia. It is unrelated to the biochemical mechanism of the complement system. **High-Yield Clinical Pearls for NEET-PG:** * **C3 Convertase Compositions:** * Classical/Lectin Pathway: **C4b2b** (formerly C4b2a). * Alternative Pathway: **C3bBb**. * **C5 Convertase:** Formed when C3b binds to C3 convertase (C4b2b3b or C3bBb3b). * **MAC (Membrane Attack Complex):** Initiated by C5b and consists of **C5b-C9**. * **Deficiency:** C3 deficiency is the most severe complement deficiency, leading to recurrent pyogenic infections and Type III hypersensitivity reactions.

Question 772: Nagler reaction is a type of:

A. Neutralization reaction (Correct Answer)
B. Complement fixation test
C. Precipitation
D. Agglutination

Explanation: **Explanation:** The **Nagler reaction** is a biochemical test used for the rapid identification of *Clostridium perfringens*. It is a classic example of a **toxin-antitoxin neutralization reaction**. 1. **Why Neutralization is correct:** *Clostridium perfringens* produces an exotoxin called **Alpha-toxin (Lecithinase)**. When the bacteria are grown on an egg yolk agar medium, the lecithinase breaks down the lecithin in the egg yolk, resulting in an opaque halo around the colonies. In the Nagler reaction, one half of the agar plate is smeared with **anti-alpha toxin (antitoxin)**. On this side, the antitoxin neutralizes the toxin, preventing the breakdown of lecithin. Therefore, opalescence appears only on the side without the antitoxin, confirming the specific activity of the toxin. 2. **Why other options are incorrect:** * **Complement Fixation Test:** This involves the consumption of complement by an antigen-antibody complex, typically used for viral or certain bacterial serology (e.g., Wassermann test). * **Precipitation:** This occurs when a soluble antigen reacts with an antibody to form an insoluble precipitate (e.g., VDRL, Elek’s test). * **Agglutination:** This involves the clumping of particulate antigens (like whole bacteria or RBCs) by antibodies (e.g., Widal test). **High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Clostridium perfringens* (Type A is the most common human pathogen). * **Target:** Alpha-toxin is a phospholipase C (lecithinase) that damages cell membranes. * **Clinical Condition:** Gas gangrene (Myonecrosis) and food poisoning. * **Other Identification:** "Stormy fermentation" in litmus milk and "Double zone of hemolysis" on blood agar.

Question 773: CD4 cells recognize antigens in association with which of the following?

A. MHC I
B. MHC II (Correct Answer)
C. MHC III
D. B-cell receptor

Explanation: **Explanation:** The recognition of antigens by T-lymphocytes is governed by the principle of **MHC Restriction**. T-cells do not recognize free-floating antigens; instead, they only recognize peptide fragments presented on **Major Histocompatibility Complex (MHC)** molecules. * **Why MHC II is correct:** CD4+ T-cells (Helper T-cells) specifically recognize exogenous antigens presented by **MHC Class II** molecules. These molecules are found exclusively on **Professional Antigen-Presenting Cells (APCs)** such as macrophages, B-cells, and dendritic cells. A helpful mnemonic is the **"Rule of 8"**: * CD**4** × MHC **II** = 8 * CD**8** × MHC **I** = 8 **Analysis of Incorrect Options:** * **A. MHC I:** These molecules are expressed on all nucleated cells and present endogenous antigens (e.g., viral or tumor proteins) to **CD8+ Cytotoxic T-cells**. * **C. MHC III:** These genes encode various components of the innate immune system, such as complement proteins (C2, C4) and cytokines (TNF-α), but they are not involved in antigen presentation to T-cells. * **D. B-cell receptor (BCR):** This is a membrane-bound antibody (IgM/IgD) on B-cells that recognizes free, native antigens directly without the need for MHC presentation. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class II Structure:** Composed of two polypeptide chains (α and β) of roughly equal size. * **Invariant Chain:** This protein prevents premature binding of self-proteins to MHC II while it is in the Endoplasmic Reticulum. * **HLA Association:** MHC II is encoded by the **HLA-DP, DQ, and DR** loci on Chromosome 6. * **Exogenous Pathway:** MHC II is associated with the endocytic pathway, where extracellular pathogens are internalized and degraded in lysosomes.

Question 774: Which of the following statements regarding superantigens is NOT true?

A. They bind directly to both MHC class II molecules and T cell receptors, causing T cell activation.
B. They bind to the cleft (antigen binding groove) in the MHC class II molecule. (Correct Answer)
C. They bind directly to the lateral aspect of the T cell receptor.
D. They bind to T cells irrespective of the antigen specificity of the T cell receptor.

Explanation: ### Explanation **Superantigens** are unique proteins produced by certain bacteria and viruses that bypass the traditional rules of antigen processing and presentation. **1. Why Option B is the Correct Answer (The False Statement):** Unlike conventional antigens, superantigens **do not bind to the peptide-binding groove (cleft)** of the MHC class II molecule. Instead, they bind to the **outer lateral surface** of the MHC II molecule and the **Vβ region** of the T-cell receptor (TCR). Because they bypass the cleft, they do not require processing by antigen-presenting cells (APCs) into peptides. **2. Analysis of Other Options:** * **Option A:** This is true. Superantigens act as a "bridge," cross-linking the MHC II on APCs and the TCR on T cells, leading to massive, non-specific activation. * **Option C:** This is true. They bind to the lateral aspect (specifically the Vβ chain) of the TCR, outside the normal antigen recognition site. * **Option D:** This is true. Because they bind externally, they activate T cells **polyclonally**, regardless of the TCR's specific antigen affinity. While normal antigens activate <0.01% of T cells, superantigens can activate up to **20-25%**, leading to a "cytokine storm" (IFN-γ, IL-1, IL-6, TNF-α). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Examples:** Staphylococcal **TSST-1** (Toxic Shock Syndrome), Streptococcal **Pyrogenic Exotoxin (SpeA/C)**, and Staphylococcal **Enterotoxins** (Food poisoning). * **MHC Association:** Superantigens specifically interact with **MHC Class II**, not Class I; therefore, they primarily activate **CD4+ T cells**. * **Consequence:** The massive release of cytokines leads to systemic inflammation, hypotension, multi-organ failure, and shock.

Question 775: Charcot Leyden crystals are derived from which type of cell?

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

Explanation: **Explanation:** **Correct Answer: B. Eosinophils** Charcot-Leyden crystals (CLCs) are hexagonal, bipyramidal, needle-like structures that serve as a hallmark of eosinophilic inflammation. They are composed of the protein **Galectin-10**, which is a lysophospholipase binding protein found in the cytoplasm of **eosinophils**. When eosinophils undergo degranulation or cell death (lysis), Galectin-10 is released and crystallizes to form these characteristic structures. **Analysis of Incorrect Options:** * **A. Macrophages:** While macrophages are involved in chronic inflammation and may phagocytose CLCs, they do not produce the Galectin-10 protein required to form them. * **C. Basophils:** Although basophils contain some Galectin-10, the concentration is significantly lower than in eosinophils. Clinically, CLCs are almost exclusively diagnostic of eosinophilic processes. * **D. Neutrophils:** Neutrophils are associated with acute bacterial inflammation and produce different markers, such as myeloperoxidase. They do not form CLCs. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Significance:** The presence of CLCs in clinical samples (sputum, stool, or tissue) indicates an **eosinophil-rich exudate**. * **Associated Conditions:** * **Bronchial Asthma:** Found in the sputum (often alongside Curschmann spirals). * **Parasitic Infections:** Found in the stool (e.g., *Entamoeba histolytica*, *Ascariasis*, or Hookworm infections). * **Allergic Rhinitis/Sinusitis:** Found in nasal secretions. * **Morphology:** They appear as slender, colorless, "double-pointed" needles that stain purplish-red with trichrome stains.

Question 776: What is the major role of HLA class III region genes?

A. Immune surveillance
B. Transplant rejection phenomenon
C. Antigen presentation and elimination
D. Governing susceptibility to autoimmune diseases (Correct Answer)

Explanation: The Human Leukocyte Antigen (HLA) complex is located on the short arm of chromosome 6. While Class I and II are primarily involved in antigen recognition, the **HLA Class III region** is unique because its gene products do not function as cell-surface markers for antigen presentation. ### 1. Why Option D is Correct The HLA Class III region encodes various proteins involved in the inflammatory response, including **Complement components (C2, C4A, C4B)**, **Tumor Necrosis Factor (TNF-α, TNF-β)**, and **Heat Shock Proteins (HSP)**. Deficiencies or polymorphisms in these specific genes (especially C4) are strongly linked to the development of systemic autoimmune conditions. For example, C4 deficiency is a major genetic risk factor for **Systemic Lupus Erythematosus (SLE)**. Therefore, the major clinical role of this region is governing susceptibility to autoimmune diseases. ### 2. Why Other Options are Incorrect * **Options A, B, and C:** These are the primary functions of **HLA Class I (A, B, C)** and **HLA Class II (DR, DQ, DP)**. Class I molecules present endogenous antigens to CD8+ T-cells, while Class II molecules present exogenous antigens to CD4+ T-cells. These interactions are the basis for immune surveillance, antigen elimination, and the T-cell-mediated response that leads to transplant rejection. ### 3. NEET-PG High-Yield Pearls * **Location:** HLA complex is on **Chromosome 6p** (short arm). * **Class III Products:** Remember the mnemonic **"C-T-H"** (Complement, TNF, HSP). It also encodes 21-hydroxylase (relevant in Congenital Adrenal Hyperplasia). * **Key Disease Association:** HLA-B27 (Class I) is associated with Ankylosing Spondylitis, but **HLA-DR4** (Class II) and **C4 null alleles** (Class III) are classic markers for Rheumatoid Arthritis and SLE susceptibility, respectively. * **Structure:** Unlike Class I and II, Class III molecules are **secreted proteins** found in the plasma, not transmembrane receptors.

Question 777: The fusion of a myeloma cell with an antigen-sensitized B cell lymphocyte is termed as?

A. Dendritic cell
B. Opsonization
C. Natural killer cell
D. Hybridoma (Correct Answer)

Explanation: **Explanation:** The correct answer is **Hybridoma**. This process is the cornerstone of the **Kohler and Milstein technique** used to produce **monoclonal antibodies (mAbs)**. 1. **Why Hybridoma is correct:** A hybridoma is a hybrid cell produced by the fusion of two different cell types: * **Antigen-sensitized B-lymphocytes:** These provide the genetic information to produce a specific antibody but have a limited lifespan (they die quickly in culture). * **Myeloma cells (Cancerous B-cells):** These are "immortal" and can divide indefinitely but lack the ability to produce the specific antibody. The resulting **Hybridoma** possesses the best of both worlds: the specificity of the B-cell and the immortality of the myeloma cell, allowing for the continuous production of identical (monoclonal) antibodies. 2. **Why other options are incorrect:** * **Dendritic cell:** These are professional antigen-presenting cells (APCs) that process and present antigens to T-cells; they are not products of cell fusion. * **Opsonization:** This is a process where pathogens are coated with substances (like IgG or C3b) to enhance their recognition and ingestion by phagocytes. * **Natural killer (NK) cell:** These are a type of cytotoxic lymphocyte critical to the innate immune system for killing virally infected or tumor cells. **High-Yield Clinical Pearls for NEET-PG:** * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select for hybridomas. Myeloma cells used are HGPRT deficient, so only fused hybrid cells can survive. * **Monoclonal Antibodies:** Examples frequently tested include **Infliximab** (anti-TNFα), **Rituximab** (anti-CD20), and **Trastuzumab** (anti-HER2). * **Nobel Prize:** Kohler and Milstein received the Nobel Prize in 1984 for this discovery.

Question 778: What is true about MHC Class I?

A. Present on chromosome 4
B. Class III has complement (Correct Answer)
C. Class II comprises A, B, C loci
D. Class I is involved in mixed leucocyte reaction

Explanation: The Major Histocompatibility Complex (MHC), known as Human Leukocyte Antigen (HLA) in humans, is a cluster of genes located on the **short arm of Chromosome 6**. It is divided into three classes based on structure and function. **Explanation of the Correct Option:** * **Option B (Correct):** The MHC Class III region is located between Class I and Class II loci. Unlike Class I and II, it does not encode antigen-presenting molecules. Instead, it encodes various secreted proteins with immune functions, most notably **complement components (C2, C4A, C4B)** and **Factor B**, as well as cytokines like TNF-α and TNF-β. **Analysis of Incorrect Options:** * **Option A:** MHC genes are located on **Chromosome 6**, not Chromosome 4. * **Option C:** The **Class I** region comprises the **A, B, and C loci**. Class II comprises the **DP, DQ, and DR** loci. * **Option D:** The **Mixed Leukocyte Reaction (MLR)** is primarily a measure of **MHC Class II** incompatibility (specifically the HLA-DR locus), which triggers T-helper cell proliferation. Class I is primarily involved in the effector phase of graft rejection. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** Found on all nucleated cells; presents endogenous antigens to **CD8+ T-cells**. * **MHC Class II:** Found only on Professional Antigen Presenting Cells (APCs) like B-cells, macrophages, and dendritic cells; presents exogenous antigens to **CD4+ T-cells**. * **Structure:** Class I consists of one heavy chain and a **β2-microglobulin** (encoded on Chromosome 15). Class II consists of two heavy chains (α and β). * **Ankylosing Spondylitis:** Strongly associated with **HLA-B27** (a Class I allele).

Question 779: C3b is converted to C3 convertase by?

A. Factor B (Correct Answer)
B. Factor P
C. Factor H
D. Factor I

Explanation: ### Explanation The question refers to the **Alternative Pathway** of the complement system. **1. Why Factor B is Correct:** The alternative pathway is unique because it is constantly active at low levels ("C3 tick-over"). When C3b is generated, it binds to **Factor B** in the presence of magnesium ions ($Mg^{2+}$). This binding forms the C3bB complex. Subsequently, Factor D cleaves Factor B into Ba and Bb. The resulting **C3bBb** complex is the functional **C3 convertase** of the alternative pathway, which then goes on to cleave more C3 into C3a and C3b, creating an amplification loop. **2. Why the Other Options are Incorrect:** * **Factor P (Properdin):** It does not convert C3b; rather, it **stabilizes** the C3 convertase (C3bBb) once it is formed, increasing its half-life. * **Factor H:** This is a **negative regulator**. It competes with Factor B for binding to C3b and acts as a cofactor for Factor I to degrade C3b, thus inhibiting the pathway. * **Factor I:** This is a protease that **inactivates C3b** (converting it to iC3b), thereby preventing the formation of C3 convertase. **3. NEET-PG Clinical Pearls:** * **C3 Convertases:** In the Classical/Lectin pathways, it is **C4b2a**; in the Alternative pathway, it is **C3bBb**. * **C5 Convertases:** Formed by adding another C3b to the existing C3 convertase (Classical: **C4b2a3b**; Alternative: **C3bBb3b**). * **Deficiency:** Deficiency of Factor H or I leads to uncontrolled C3 consumption, resulting in low C3 levels and increased susceptibility to pyogenic infections. * **Properdin** is the only known positive regulator of complement activation.

Question 780: Interleukin-1 (IL-1) is secreted by which of the following cells?

A. Neutrophils
B. Macrophages
C. Epithelial cells
D. All of the above (Correct Answer)

Explanation: **Explanation:** Interleukin-1 (IL-1) is a pivotal pro-inflammatory cytokine that plays a central role in the body's innate immune response. It exists primarily in two forms: IL-1α and IL-1β. **Why "All of the above" is correct:** While **Macrophages** (and Monocytes) are the primary and most potent producers of IL-1, the production of this cytokine is not exclusive to them. * **Macrophages:** Secrete IL-1 in response to Pathogen-Associated Molecular Patterns (PAMPs) like LPS via the activation of the inflammasome. * **Neutrophils:** Actively produce IL-1 during acute inflammation to amplify the recruitment of more leucocytes. * **Epithelial cells:** (including keratinocytes) store and release IL-1 as an "alarmin" when the physical barrier is damaged or infected. **Analysis of Options:** * **A & B (Neutrophils & Macrophages):** These are professional phagocytes and the first line of defense. They utilize the NLRP3 inflammasome to process pro-IL-1 into its active form. * **C (Epithelial cells):** Non-immune cells like epithelial cells, endothelial cells, and fibroblasts also secrete IL-1 to signal local tissue distress to the systemic immune system. **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Pyrogen:** IL-1 is a potent endogenous pyrogen. It acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2), raising the thermoregulatory set-point (causing fever). * **Acute Phase Response:** Along with IL-6 and TNF-α, IL-1 stimulates the liver to produce acute-phase proteins (e.g., CRP, Fibrinogen). * **Clinical Correlation:** **Anakinra** is a recombinant IL-1 receptor antagonist used in the treatment of Rheumatoid Arthritis and Cryopyrin-Associated Periodic Syndromes (CAPS). * **Adhesion:** IL-1 increases the expression of adhesion molecules (E-selectin, ICAM-1) on vascular endothelium, facilitating leucocyte extravasation.

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