Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 241: Immunoglobulin bound on the surface of bacteria mediates phagocytosis by:

A. C3b and Fc (Correct Answer)
B. Receptor mediated endocytosis
C. Oxidase action
D. Lysosomal burst

Explanation: This question tests the concept of **Opsonization**, the process by which pathogens are marked for ingestion and destruction by phagocytes (macrophages and neutrophils). ### **Explanation of the Correct Answer** Opsonization acts like a "molecular handle" that allows phagocytes to grip slippery bacterial capsules. The two most potent opsonins in the human body are **IgG** and the complement fragment **C3b**. * **Fc Receptor:** Phagocytes possess surface receptors for the **Fc portion of IgG** (specifically IgG1 and IgG3). When IgG binds to a bacterial antigen via its Fab portion, the exposed Fc tail binds to these receptors. * **C3b Receptor (CR1):** The activation of the complement pathway leads to the deposition of C3b on the bacterial surface. Phagocytes express **CR1 receptors** that bind to C3b. * **Synergy:** While either can work independently, the binding of both C3b and the Fc portion of IgG leads to a synergistic effect, significantly enhancing the efficiency of phagocytosis. ### **Why Other Options are Incorrect** * **B. Receptor-mediated endocytosis:** This is a general cellular process for internalizing specific molecules (like LDL or iron). While phagocytosis is a form of endocytosis, it is a distinct, large-scale process triggered specifically by opsonins in the context of immunity. * **C & D. Oxidase action / Lysosomal burst:** These processes occur **after** the bacterium has already been engulfed. They are mechanisms of intracellular killing (Respiratory Burst) rather than the mechanism of binding or mediation of ingestion. ### **High-Yield Clinical Pearls for NEET-PG** * **Mnemonic for Opsonins:** "**O**psonization **P**repares **S**ubstances **O**nly **N**ow" (**I**gG and **C**3b). * **IgG vs. IgM:** IgG is an opsonin because phagocytes have Fc receptors for it. **IgM is NOT an opsonin** directly because phagocytes lack Fc receptors for IgM; however, IgM is a potent activator of complement, leading to C3b deposition. * **Splenectomy:** Patients without a spleen are at risk for encapsulated bacteria (e.g., *S. pneumoniae*) because they lack the splenic macrophages required to clear IgG/C3b-coated pathogens.

Question 242: Which of the following is NOT a peripheral lymphoid organ?

A. Lymph nodes
B. Spleen
C. Mucosa-associated lymphoid tissue
D. Thymus (Correct Answer)

Explanation: ### Explanation The lymphoid system is divided into two functional categories: **Primary (Central)** and **Secondary (Peripheral)** lymphoid organs. **Why Thymus is the Correct Answer:** The **Thymus** (along with Bone Marrow) is a **Primary Lymphoid Organ**. These are the sites where lymphocytes are produced and undergo antigen-independent maturation. In the thymus, T-cell precursors from the bone marrow differentiate into mature, immunocompetent T-cells. Since the question asks for an organ that is *NOT* peripheral, the Thymus is the correct choice. **Why the other options are incorrect:** Secondary (Peripheral) lymphoid organs are sites where mature lymphocytes interact with antigens and undergo clonal expansion. * **A. Lymph nodes:** These filter lymph and are the primary sites for initiating immune responses to tissue-borne antigens. * **B. Spleen:** This is the largest peripheral lymphoid organ; it filters blood and responds to blood-borne antigens. * **C. Mucosa-associated lymphoid tissue (MALT):** This includes Peyer’s patches, tonsils, and appendix. They protect mucosal surfaces (respiratory, GI, and urogenital tracts). **High-Yield Clinical Pearls for NEET-PG:** * **Primary Lymphoid Organs:** Bone Marrow (B-cell maturation) and Thymus (T-cell maturation). * **Hassall’s Corpuscles:** These are characteristic histological features found in the medulla of the **Thymus**. * **DiGeorge Syndrome:** A congenital defect (3rd and 4th pharyngeal pouches) leading to thymic hypoplasia and T-cell deficiency. * **MALT:** Specifically, **Peyer’s patches** in the ileum are characterized by M-cells that sample intestinal antigens. * **Major site of antibody production:** The Spleen (specifically the white pulp) and Lymph nodes.

Question 243: The hybridoma technique is used to obtain which of the following?

A. Specific antigen
B. Complement
C. Specific antibody (Correct Answer)
D. Interleukins

Explanation: ### Explanation The **Hybridoma Technique**, pioneered by Georges Köhler and César Milstein in 1975, is the gold-standard method for producing **Monoclonal Antibodies (mAbs)**. These are highly specific antibodies derived from a single clone of B-cells, ensuring they all recognize the same epitope on an antigen. **Why Option C is Correct:** The technique involves fusing short-lived, antibody-producing **B-lymphocytes** (usually from a mouse spleen) with immortal **Myeloma cells** (cancerous B-cells). The resulting "Hybridoma" cell inherits two key properties: 1. **Immortality** from the myeloma cell (allowing continuous growth in culture). 2. **Specificity** from the B-lymphocyte (allowing the production of a single, specific antibody). **Why Other Options are Incorrect:** * **A. Specific Antigen:** Antigens are the targets that trigger the immune response; they are used to immunize the animal before the fusion process, not produced by it. * **B. Complement:** These are heat-labile plasma proteins synthesized primarily by the liver and macrophages, not by hybridoma cells. * **D. Interleukins:** These are cytokines produced by various leukocytes (like T-cells and macrophages) to modulate the immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select only the fused hybrid cells. Unfused myeloma cells die because Aminopterin blocks the *de novo* pathway, and they lack the **HGPRT enzyme** for the salvage pathway. * **Applications:** Monoclonal antibodies are used in **ELISA**, **Western Blotting**, and as therapeutic agents (e.g., **Rituximab** against CD20, **Infliximab** against TNF-α). * **Nobel Prize:** Köhler and Milstein received the Nobel Prize in 1984 for this discovery.

Question 244: Which of the following cytokines do not play a role in the production, maintenance, or activation of lymphocytes?

A. IL-1
B. IL-2
C. IL-3
D. IL-12 (Correct Answer)

Explanation: ### Explanation The correct answer is **IL-12**. While IL-12 is a critical cytokine in the immune response, its primary function is the **differentiation** of naive T-cells into Th1 cells and the activation of Natural Killer (NK) cells. It acts as a bridge between innate and adaptive immunity rather than being a primary driver for the production (hematopoiesis), homeostatic maintenance, or general activation of the lymphocyte pool itself. **Why the other options are incorrect:** * **IL-1:** Produced by macrophages, it is a "pro-inflammatory" cytokine that acts as a costimulator for **T-cell activation** and promotes B-cell proliferation. * **IL-2:** Known as the **T-cell growth factor**, it is essential for the proliferation (clonal expansion) and maintenance of T-cells. It is the most critical cytokine for lymphocyte survival. * **IL-3:** Acts as a **multilineage colony-stimulating factor (m-CSF)**. It is produced by activated T-cells to support the **production** of all hematopoietic cells, including lymphoid progenitors, from the bone marrow. **High-Yield Clinical Pearls for NEET-PG:** * **IL-2 Clinical Link:** Aldesleukin (recombinant IL-2) is used in the treatment of Renal Cell Carcinoma and Melanoma. * **IL-12 Deficiency:** Leads to disseminated mycobacterial infections (e.g., BCG-osis) because the body cannot mount a Th1/IFN-$\gamma$ response. * **Hot T-Bone stEAK (Mnemonic for Cytokines):** * **IL-1:** Fever (**Hot**) * **IL-2:** Stimulates **T**-cells * **IL-3:** Stimulates **Bone** marrow * **IL-4:** Stimulates Ig**E** production * **IL-5:** Stimulates Ig**A** production * **IL-6:** Stimulates a**K**ute (acute) phase reactants

Question 245: Which of the following reactions is NOT produced by heterophile antigens?

A. Forssman antigen-antibody reaction
B. Weil-Felix reaction
C. Paul-Bunnel reaction
D. Widal reaction (Correct Answer)

Explanation: ### Explanation **Concept of Heterophile Antigens** Heterophile antigens are closely related antigens present in completely different species (phylogenetically unrelated). An antibody produced against one of these antigens can cross-react with the antigen of another species. This principle is used in various diagnostic serological tests. **Why the Widal Reaction is the Correct Answer** The **Widal reaction** is a **specific** agglutination test used for the diagnosis of Enteric fever (Typhoid). It detects antibodies against the specific O (somatic) and H (flagellar) antigens of *Salmonella typhi* and *S. paratyphi*. Since it involves specific homologous antigen-antibody interactions rather than cross-reactive antigens from different species, it is **not** a heterophile reaction. **Analysis of Incorrect Options** * **Forssman Antigen-Antibody Reaction:** This is the classic example of a heterophile system. Forssman antigens are found in the tissues of horses, guinea pigs, and cats; antibodies against them cross-react with various other species. * **Weil-Felix Reaction:** A heterophile agglutination test where antibodies produced during **Rickettsial infections** cross-react with the alkali-stable polysaccharide antigens of certain **Proteus strains** (OX19, OX2, and OXK). * **Paul-Bunnel Reaction:** Used to diagnose **Infectious Mononucleosis** (EBV). It detects heterophile antibodies in the patient's serum that have the unique property of agglutinating **sheep erythrocytes**. **High-Yield Clinical Pearls for NEET-PG** * **Paul-Bunnel Test:** Specific for EBV; uses sheep RBCs. * **Weil-Felix Test:** Uses *Proteus* antigens to diagnose Rickettsial diseases (except Q fever). * **Cold Agglutination Test:** Another heterophile test where antibodies in *Mycoplasma pneumoniae* patients agglutinate human O-group RBCs at 4°C. * **Widal Test:** A "TUBE" agglutination test; significant titers are usually >1:80 for O and >1:160 for H.

Question 246: The Postzone phenomenon is seen in which condition?

A. Antigen excess (Correct Answer)
B. Antibody excess
C. Equivalence zone
D. None of the above

Explanation: The **Postzone phenomenon** occurs during an antigen-antibody reaction when there is a relative **excess of antigen**. This prevents the formation of large, visible lattices (precipitates or agglutinates), leading to a false-negative result despite the presence of antibodies. ### Explanation of Options: * **A. Antigen excess (Correct):** In the postzone, the amount of antigen is so high that each antibody molecule is saturated by individual antigens. This prevents the cross-linking required to form a stable lattice structure. * **B. Antibody excess:** This is known as the **Prozone phenomenon**. Here, every antigenic site is covered by an excess of antibodies, preventing the bridging of antigens and resulting in a false-negative test. This is clinically significant in secondary syphilis (VDRL/RPR) and Brucellosis. * **C. Equivalence zone:** This is the ideal ratio where the concentration of antigen and antibody is optimal for maximum lattice formation and visible precipitation. ### High-Yield Clinical Pearls for NEET-PG: * **Marrack’s Lattice Hypothesis:** Explains that for a visible reaction, both antigen and antibody must be multivalent and present in optimal proportions. * **Clinical Significance:** If a clinician suspects a disease (e.g., Syphilis) but the screening test is negative, the serum should be **diluted** to overcome the Prozone effect. * **Prozone vs. Postzone:** Prozone (Antibody excess) is more common in clinical practice than Postzone (Antigen excess). * **Key mnemonic:** **P**rozone = **P**receding (excess antibody at the start); **P**ostzone = **P**ost-equivalence (excess antigen added).

Question 247: Toll-like receptors are expressed by which of the following cell types?

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

Explanation: **Explanation:** **Toll-like receptors (TLRs)** are a class of **Pattern Recognition Receptors (PRRs)** that play a critical role in the innate immune system. They recognize highly conserved structural motifs known as **Pathogen-Associated Molecular Patterns (PAMPs)**, such as LPS, flagellin, and viral double-stranded RNA. 1. **Why "All the above" is correct:** While TLRs are most famously associated with professional **sentinel cells** like **Macrophages** and Dendritic cells (which use them to trigger phagocytosis and cytokine release), they are not exclusive to immune cells. * **Epithelial cells** (e.g., in the gut and lungs) express TLRs to act as a first line of defense at mucosal barriers. * **Fibroblasts** express TLRs to sense tissue injury and infection, contributing to the inflammatory response and wound healing. * They are also found on B-cells, neutrophils, and even certain endothelial cells. 2. **Analysis of Options:** * **Option A (Macrophages):** Correct, but incomplete. They express a wide array of TLRs (like TLR4 for LPS) to initiate systemic inflammation. * **Option B (Epithelial cells):** Correct, but incomplete. They use TLRs to maintain barrier integrity and signal the underlying immune system. * **Option C (Fibroblasts):** Correct, but incomplete. They produce chemokines upon TLR activation to recruit leukocytes to the site of infection. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** TLRs are found on the **cell surface** (TLR 1, 2, 4, 5, 6) or in **endosomes** (TLR 3, 7, 8, 9). * **TLR-4:** Specifically recognizes **Lipopolysaccharide (LPS)** of Gram-negative bacteria. * **TLR-3:** Recognizes **dsRNA** (viral). * **TLR-5:** Recognizes **Flagellin**. * **Adapter Protein:** Most TLRs (except TLR3) use the **MyD88** signaling pathway to activate **NF-κB**, leading to cytokine production.

Question 248: Which of the following is NOT a C-C chemokine?

A. IL-18 (Correct Answer)
B. MCP-1
C. Eotaxin
D. MIP-1a

Explanation: ### Explanation **Correct Answer: A. IL-18** **Why IL-18 is the correct answer:** Chemokines are a superfamily of small cytokines (8–10 kDa) characterized by their ability to induce chemotaxis. They are classified into four families based on the arrangement of conserved N-terminal cysteine residues: **CC, CXC, C, and CX3C**. **IL-18** (Interferon-gamma inducing factor) is a member of the **IL-1 family of cytokines**, not a chemokine. It plays a role in the innate immune response by stimulating T-cells and NK cells to produce IFN-γ. It lacks the structural cysteine motifs characteristic of the chemokine family. **Analysis of Incorrect Options:** * **B. MCP-1 (Monocyte Chemoattractant Protein-1):** Also known as **CCL2**, it is a classic CC chemokine. It is responsible for recruiting monocytes, memory T cells, and dendritic cells to sites of inflammation. * **C. Eotaxin:** Also known as **CCL11**, it is a CC chemokine that selectively recruits eosinophils. It is highly relevant in allergic conditions like asthma. * **D. MIP-1α (Macrophage Inflammatory Protein-1 alpha):** Also known as **CCL3**, it is a CC chemokine involved in the recruitment and activation of polymorphonuclear leukocytes. **High-Yield Facts for NEET-PG:** 1. **CC Chemokines (β-chemokines):** The first two cysteines are adjacent. Examples include **MIP, MCP, RANTES, and Eotaxin**. They primarily act on monocytes, lymphocytes, and eosinophils. 2. **CXC Chemokines (α-chemokines):** One amino acid separates the first two cysteines. The most important example is **IL-8 (CXCL8)**, which is the primary chemoattractant for **neutrophils**. 3. **Clinical Correlation:** **CCR5** (a CC chemokine receptor) serves as a co-receptor for **HIV-1** entry into macrophages. A mutation in this receptor (CCR5-Δ32) provides resistance to HIV infection.

Question 249: What is the smallest unit of antigenicity?

A. Epitope (Correct Answer)
B. Paratope
C. Idiotypic
D. Hapten

Explanation: **Explanation:** The correct answer is **A. Epitope**. **Why Epitope is correct:** An **epitope**, also known as an antigenic determinant, is the specific chemical group or molecular configuration on the surface of an antigen that is recognized by the immune system (specifically by antibodies, B-cells, or T-cells). It is considered the **smallest functional unit of antigenicity**. A single antigen molecule can possess multiple different epitopes, each capable of stimulating a distinct immune response. **Analysis of Incorrect Options:** * **B. Paratope:** This is the antigen-binding site located on the **antibody** (specifically the hypervariable regions of the Fab fragment). While the epitope is on the antigen, the paratope is its corresponding "lock" on the antibody. * **C. Idiotypic:** This refers to the unique antigenic determinants found within the variable domains of antibodies or T-cell receptors. It characterizes the specificity of a single clone of cells. * **D. Hapten:** These are low-molecular-weight substances that are **antigenic but not immunogenic**. They can only induce an immune response when conjugated to a larger carrier protein. While small, a hapten is a type of molecule, whereas an epitope is the specific site of recognition. **High-Yield Clinical Pearls for NEET-PG:** * **T-cell vs. B-cell Epitopes:** B-cell epitopes are usually hydrophilic and can be conformational (dependent on 3D shape) or linear. T-cell epitopes are always linear peptides processed and presented via MHC molecules. * **Valency:** The number of epitopes on an antigen surface is called its valency. Most natural antigens are multivalent. * **Adjuvants:** Substances added to vaccines to enhance the immunogenicity of an antigen without being antigenic themselves (e.g., Alum).

Question 250: All are toxin-antitoxin neutralization tests except:

A. Schick test
B. Eleks gel precipitation test (Correct Answer)
C. Neil Mooser reaction
D. Naeglers test

Explanation: ### Explanation The core concept of this question lies in distinguishing between **neutralization tests** (where an antitoxin inhibits a toxin's biological activity) and **precipitation tests** (where an antigen and antibody react to form a visible physical complex). **Why Option B is Correct:** The **Elek’s gel precipitation test** is an *in vitro* **immunodiffusion (precipitation) test** used to detect the toxigenicity of *Corynebacterium diphtheriae*. While it involves toxin and antitoxin, the principle is the formation of a visible white line of precipitation where they meet in optimal proportions in agar, rather than the neutralization of a biological effect. **Analysis of Incorrect Options:** * **A. Schick test:** An *in vivo* neutralization test used to determine immunity against Diphtheria. Toxin is injected intradermally; if the patient has circulating antitoxin, the toxin is **neutralized**, and no reaction occurs (Negative test). * **C. Neil-Mooser (Tunica Vaginalis) reaction:** A neutralization/identification test used to differentiate *Rickettsia typhi* (Endemic typhus) from *R. prowazekii*. It involves the observation of scrotal swelling in guinea pigs, which can be inhibited by specific antisera. * **D. Nagler’s test:** A neutralization test used to identify *Clostridium perfringens*. It detects **Alpha-toxin (Lecithinase)** activity. The toxin produces opalescence on egg yolk agar, which is **inhibited (neutralized)** on the half of the plate containing antitoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Diphtheria:** Schick test = *In vivo* neutralization; Elek’s test = *In vitro* precipitation. * **Nagler’s Reaction:** Rapid identification of *C. perfringens*; look for "Lecithinase" activity. * **ASO (Antistreptolysin O) Titer:** Another classic example of a neutralization test (toxin-antitoxin) used in clinical practice for Rheumatic fever.

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Immunology — MCQs

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