Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 731: Which of the following is an example of Type IV hypersensitivity?

A. Arthus reaction
B. Serum sickness
C. Shwartzman reaction
D. Granulomatous reaction (Correct Answer)

Explanation: **Explanation:** **Type IV Hypersensitivity (Delayed-type)** is a cell-mediated immune response involving T-lymphocytes (CD4+ Th1 cells and CD8+ cytotoxic T cells) rather than antibodies. **Why the Correct Answer is Right:** **D. Granulomatous reaction:** This is the classic example of Type IV hypersensitivity. It occurs when an antigen (like *M. tuberculosis*) persists within macrophages because it cannot be easily eliminated. Th1 cells secrete cytokines (IFN-γ), which activate macrophages, transforming them into epithelioid cells and multinucleated giant cells, eventually forming a granuloma. This process typically takes 48–72 hours or longer to develop. **Why the Other Options are Incorrect:** * **A. Arthus reaction:** This is a localized **Type III hypersensitivity** reaction. It involves the formation of immune complexes (antigen-antibody) in situ, leading to complement activation and necrotizing vasculitis. * **B. Serum sickness:** This is a systemic **Type III hypersensitivity** reaction. It occurs when circulating immune complexes deposit in tissues (joints, kidneys, vessels) following the administration of foreign serum or drugs. * **C. Shwartzman reaction:** This is **not a hypersensitivity reaction**. It is an exaggerated inflammatory response to endotoxins (LPS), characterized by localized or systemic hemorrhagic necrosis and intravascular coagulation. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Type IV:** "4 T's" – **T**-cells, **T**ransplant (Rejection), **T**B test (Mantoux), **T**ouch (Contact dermatitis). * **Other Examples:** Lepromin test, Nickel allergy, and Graft-versus-host disease (GVHD). * **Key Cytokine:** **Interferon-gamma (IFN-γ)** is the most critical cytokine for macrophage activation in Type IV reactions.

Question 732: Opsonization occurs due to all of the following except?

A. Endotoxin (Correct Answer)
B. Complement
C. IgM
D. IgD

Explanation: ### Explanation **Opsonization** is the process by which pathogens are coated with specific molecules (opsonins) to enhance their recognition and ingestion by phagocytes (neutrophils and macrophages). **1. Why Endotoxin is the Correct Answer:** Endotoxin (Lipopolysaccharide/LPS), found in the outer membrane of Gram-negative bacteria, is a **Pathogen-Associated Molecular Pattern (PAMP)**. While it triggers the immune response by binding to Toll-like receptors (TLR-4), it does not act as an opsonin itself. Instead, it is the *target* that opsonins bind to. Therefore, it does not facilitate opsonization; it is a trigger for inflammation and potential septic shock. **2. Analysis of Other Options:** * **Complement (C3b):** C3b is the most potent opsonin in the complement system. Phagocytes have CR1 receptors that bind specifically to C3b coated on surfaces. * **IgM:** While IgM is a poor opsonin directly (due to the lack of Fc receptors for IgM on phagocytes), it is the most efficient activator of the **Classical Complement Pathway**. By activating complement, it leads to the deposition of C3b, thereby facilitating opsonization. * **IgG (specifically IgG1 and IgG3):** These are the primary antibodies that act as opsonins because phagocytes possess Fcγ receptors. * *Note on IgD:* While IgD's role is primarily as a B-cell surface receptor, in the context of this question, **Endotoxin** is the most definitive "non-opsonin" as it is a bacterial component, not a host-derived coating molecule. **3. High-Yield Clinical Pearls for NEET-PG:** * **Major Opsonins:** IgG (specifically the Fc portion) and C3b. * **Mnemonic:** **"I**t **G**oops **G**erms" for **IgG** and **C3b**. * **Acute Phase Reactant:** C-Reactive Protein (CRP) also acts as an opsonin. * **Receptors:** Phagocytosis is enhanced when both FcγR and CR1 receptors are engaged simultaneously.

Question 733: A rabbit is injected for the first time with ova-albumin. The antibodies generated in this response would be of which class?

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

Explanation: ### Explanation The correct answer is **B. IgM**. **1. Why IgM is the correct answer:** This question tests the concept of the **Primary Immune Response**. When an antigen (like ova-albumin) is introduced into a host for the very first time, the immune system undergoes a primary response. The first immunoglobulin class to be synthesized by naive B cells following initial antigenic challenge is always **IgM**. This is because IgM can be produced without the need for T-cell-mediated "isotype switching." It serves as the body's first line of adaptive humoral defense. **2. Why the other options are incorrect:** * **A. IgG:** This is the predominant antibody in the **Secondary (Anamnestic) Immune Response**. While IgG is eventually produced after the primary response via class switching, it is not the *initial* antibody generated. * **C. IgD:** This class primarily acts as a surface receptor on mature B cells. It is not secreted in significant quantities during a systemic immune response to an injected antigen. * **D. IgE:** This antibody is specifically associated with Type I Hypersensitivity reactions and parasitic infections. It is not the default primary response antibody for a general protein antigen like albumin. **3. High-Yield NEET-PG Pearls:** * **Primary Response:** Characterized by a long lag phase (5–10 days), low antibody titer, and **IgM** predominance. * **Secondary Response:** Characterized by a short lag phase (1–3 days), high antibody titer, and **IgG** predominance due to memory B cells. * **Structure:** IgM is a **pentamer** (highest valency) in secretion, making it highly efficient at agglutination and complement activation, despite having lower affinity than IgG. * **Fetal Infection:** Since IgM cannot cross the placenta, its presence in a newborn indicates an *in utero* (congenital) infection.

Question 734: The protection against smallpox by previous infection with cowpox represents which of the following?

A. Antigenic cross-reactivity (Correct Answer)
B. Antigenic specificity
C. Passive immunity
D. Innate immunity

Explanation: **Explanation:** The correct answer is **Antigenic cross-reactivity**. This phenomenon occurs when two different organisms share similar or identical epitopes (antigenic determinants). In this classic historical example, the **Vaccinia virus** (cowpox) shares structural antigens with the **Variola virus** (smallpox). When a person is infected with cowpox, the immune system produces antibodies and T-cells that recognize these shared antigens. Because of this cross-reactivity, the immune response generated against the milder cowpox virus effectively neutralizes the more virulent smallpox virus. This principle formed the basis of Edward Jenner’s first vaccine. **Analysis of Incorrect Options:** * **Antigenic specificity:** This refers to the ability of the immune system to distinguish between even minor differences in chemical structure. If the immune response were strictly specific without cross-reactivity, cowpox antibodies would ignore the smallpox virus. * **Passive immunity:** This involves the transfer of pre-formed antibodies (e.g., via placenta or immunoglobulin shots). Protection from cowpox infection is **active immunity**, as the individual’s own immune system produces the response. * **Innate immunity:** This is the non-specific, first line of defense (e.g., skin, phagocytes). Protection via prior infection involves **adaptive immunity**, which is characterized by memory and specificity. **NEET-PG High-Yield Pearls:** * **Heterologous Vaccine:** A vaccine that uses a live organism that is different from the pathogen but shares cross-reacting antigens (e.g., Cowpox for Smallpox, BCG for Tuberculosis). * **Weil-Felix Reaction:** Another clinical example of cross-reactivity where antibodies against *Rickettsia* cross-react with *Proteus* antigens (*OX-19, OX-2, OX-K*). * **Molecular Mimicry:** A form of cross-reactivity where microbial antigens resemble self-antigens, leading to autoimmunity (e.g., Rheumatic fever following *Streptococcus pyogenes* infection).

Question 735: Which bacterium is frequently used as an immunomodulator?

A. Corynebacterium parvum (Correct Answer)
B. Mycobacterium marinum
C. Chromobacterium violasium
D. Flavobacterium meningosepticum

Explanation: **Explanation:** **Corynebacterium parvum** (also known as *Propionibacterium acnes*) is a potent **non-specific immunomodulator**. It acts primarily by stimulating the reticuloendothelial system, leading to the activation of macrophages and natural killer (NK) cells. In clinical oncology and experimental immunology, it has been used as an adjuvant to enhance the immune response against tumors and to increase resistance to various bacterial and viral infections. **Analysis of Options:** * **A. Corynebacterium parvum (Correct):** It is a classic example of a bacterial immunomodulator (along with BCG). It induces cytokine production (like IL-12 and IFN-γ), which shifts the immune response toward a Th1 phenotype. * **B. Mycobacterium marinum:** This is a non-tuberculous mycobacterium (NTM) responsible for "Fish Tank Granuloma." While *Mycobacterium bovis* (BCG) is used as an immunomodulator, *M. marinum* is strictly a pathogen. * **C. Chromobacterium violaceum:** A rare opportunistic pathogen found in tropical soil and water, known for producing a violet pigment called violacein. It is not used for immunomodulation. * **D. Flavobacterium meningosepticum (now Elizabethkingia):** An environmental gram-negative rod known for causing neonatal meningitis and nosocomial outbreaks; it is highly multidrug-resistant and has no role in immunomodulation. **NEET-PG High-Yield Pearls:** * **Bacterial Immunomodulators:** The two most frequently tested are **BCG** (used intravesically for bladder cancer) and **Corynebacterium parvum**. * **Mechanism:** They act as "biological response modifiers" by stimulating innate immunity. * **Levamisole:** An anti-helminthic drug often mentioned alongside these bacteria as a chemical immunomodulator.

Question 736: In which zone are large antigen-antibody complexes formed?

A. Prozone
B. Postzone
C. Zone of equivalence (Correct Answer)
D. None of the above

Explanation: ### Explanation The formation of antigen-antibody (Ag-Ab) complexes is governed by the **Marrack’s Lattice Hypothesis**, which describes how the ratio of antigens to antibodies determines the size and visibility of the resulting precipitate. **1. Why the Zone of Equivalence is Correct:** In the **Zone of Equivalence**, the concentration of antigen and antibody is optimal (roughly equal). Each antibody molecule can bridge two different antigen molecules, and each multivalent antigen can be bound by multiple antibodies. This leads to the formation of a **large, stable, multi-molecular lattice** that becomes insoluble and visible as a precipitate. This is the basis for most precipitation-based serological assays. **2. Why the Other Options are Incorrect:** * **Prozone (Antibody Excess):** Here, the concentration of antibody is very high. Each antigen site is rapidly saturated by individual antibody molecules, preventing the cross-linking required to form a lattice. Only small, soluble complexes are formed, leading to a **false-negative** result. * **Postzone (Antigen Excess):** Here, the antigen concentration exceeds the antibody. Every antibody binding site is occupied by a single antigen molecule, leaving no room for bridging. This also results in small, soluble complexes and a **false-negative** result. **3. NEET-PG High-Yield Pearls:** * **Clinical Significance:** The Prozone phenomenon is classically seen in **Secondary Syphilis (VDRL/RPR tests)** and **Brucellosis**. If a clinical suspicion is high but the test is negative, the serum should be diluted to reach the zone of equivalence. * **Precipitation vs. Agglutination:** Precipitation involves **soluble** antigens, while agglutination involves **particulate/insoluble** antigens (like RBCs or bacteria). * **Immunodiffusion:** Techniques like Radial Immunodiffusion (Mancini) and Double Diffusion (Ouchterlony) rely on the reactants diffusing through a gel until they reach the zone of equivalence to form a visible precipitin line.

Question 737: The Frei test is used to identify which type of hypersensitivity reaction?

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

Explanation: **Explanation:** The **Frei test** is a skin test historically used for the diagnosis of **Lymphogranuloma Venereum (LGV)**, caused by *Chlamydia trachomatis* (serotypes L1, L2, and L3). It is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**. 1. **Why Type IV is correct:** The test involves the intradermal injection of the "Frei antigen" (inactivated LGV agent). In a positive case, sensitized T-lymphocytes recognize the antigen, leading to the release of cytokines and the recruitment of macrophages. This results in an inflammatory papule or induration at the injection site, which peaks at **48 to 72 hours**—the hallmark timing of a cell-mediated (Type IV) response. 2. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Atopy). These occur within minutes. * **Type II (Cytotoxic):** Mediated by IgG/IgM against cell surface antigens (e.g., Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-complex):** Involves deposition of antigen-antibody complexes in tissues (e.g., SLE, Post-streptococcal glomerulonephritis). **High-Yield Clinical Pearls for NEET-PG:** * **Status:** The Frei test is now largely obsolete, replaced by more sensitive Nucleic Acid Amplification Tests (NAAT) and serology. * **Cross-reactivity:** It is not highly specific as it can show cross-reactivity with other Chlamydia species (e.g., *C. psittaci*). * **Other Type IV Skin Tests:** Tuberculin (Mantoux) test, Lepromin test, Casoni’s test (immediate and delayed), and Montenegro test (Leishmaniasis). * **LGV Clinical Sign:** Look for the **"Groove sign"** (enlargement of inguinal lymph nodes above and below the inguinal ligament).

Question 738: Which immunoglobulin is primarily responsible for complement-mediated lysis?

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

Explanation: ### Explanation **1. Why IgM is the Correct Answer:** IgM is the most potent activator of the **Classical Complement Pathway**. Its high efficiency stems from its **pentameric structure** (five monomer units joined by a J-chain). For the first component of the complement (C1q) to be activated, it must bind to at least two Fc portions of antibodies. Because IgM is a pentamer, a single molecule of IgM bound to an antigen provides multiple closely spaced Fc sites, easily triggering the complement cascade leading to the formation of the **Membrane Attack Complex (MAC)** and subsequent cell lysis. In contrast, IgG requires at least two separate molecules to be in close proximity to achieve the same effect. **2. Why Other Options are Incorrect:** * **IgE:** Primarily involved in Type I hypersensitivity (allergic) reactions and defense against helminthic parasites. It binds to mast cells and basophils via high-affinity Fcε receptors. * **IgA:** The primary secretory antibody found in colostrum, saliva, and mucosal surfaces. It is a poor activator of the classical complement pathway; it primarily neutralizes pathogens and prevents mucosal attachment. * **IgD:** Found mainly on the surface of B-cells as an antigen receptor. Its precise systemic function is less defined, but it does not play a significant role in complement-mediated lysis. **3. Clinical Pearls for NEET-PG:** * **Potency:** IgM is **100–1000 times** more effective than IgG in mediating complement-dependent hemolysis. * **IgG Subclasses:** Among IgG, the order of complement activation efficiency is **IgG3 > IgG1 > IgG2**. IgG4 does not activate complement. * **Structure:** IgM is a pentamer in secretions/serum but a monomer when acting as a B-cell receptor (BCR). * **Nature:** IgM is the first antibody produced in response to an infection (Primary immune response) and indicates acute infection.

Question 739: Which of the following statements about MHC is false?

A. MHC class I is present on all nucleated cells.
B. MHC class I molecules are presented to CD8 T cells. (Correct Answer)
C. MHC class I antigen presentation involves the cytosolic pathway.
D. MHC class I peptides are typically 8-10 amino acids long.

Explanation: **Explanation** The question asks for the **false** statement regarding MHC molecules. However, based on immunological principles, **Option B is actually a true statement.** In NEET-PG and similar exams, if all options appear true, one must re-examine the phrasing or identify the "most" correct/incorrect fact. In this specific question, all four options are technically correct descriptions of MHC Class I. *Note: If this were a "False" question, there may be a typographical error in the provided key or options. Let’s analyze the facts:* 1. **MHC Class I Distribution (Option A):** This is **True**. MHC I is expressed on all nucleated cells and platelets. It is notably absent on mature Red Blood Cells (RBCs) because they lack a nucleus. 2. **MHC Class I Presentation (Option B):** This is **True**. MHC I molecules present endogenous antigens to **CD8+ Cytotoxic T cells**. (Mnemonic: Rule of 8; 1 x 8 = 8). 3. **Processing Pathway (Option C):** This is **True**. MHC I uses the **Endogenous/Cytosolic pathway**. Proteins in the cytosol are degraded by proteasomes, transported to the ER via TAP (Transporter associated with Antigen Processing), and loaded onto MHC I. 4. **Peptide Length (Option D):** This is **True**. The binding groove of MHC I is "closed," limiting the peptide size to typically **8–10 amino acids**. (MHC II has an "open" groove for longer peptides, 13–18+ amino acids). **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD4+ cells see MHC II; CD8+ cells see MHC I. * **Structure:** MHC I consists of a heavy chain and a **$\beta_2$-microglobulin** (encoded on Chromosome 15). MHC II consists of $\alpha$ and $\beta$ chains (both encoded on Chromosome 6). * **HLA Associations:** B27 (Ankylosing Spondylitis), DR3/DR4 (Type 1 Diabetes), DQ2/DQ8 (Celiac Disease). * **Non-nucleated cells:** RBCs do not express MHC I, which is why they cannot be infected by viruses that require MHC-mediated processes, but they can be infected by Plasmodium.

Question 740: Which condition exhibits a mechanism similar to the Schwartzman reaction?

A. Fitz Hugh Curtis syndrome
B. Waterhouse Friderichsen syndrome (Correct Answer)
C. Eichwald-Silberberg phenomenon
D. Anaphylactoid reaction

Explanation: **Explanation:** The **Shwartzman reaction** is a phenomenon of severe tissue necrosis and disseminated intravascular coagulation (DIC) triggered by repeated exposure to bacterial endotoxins (LPS). It occurs in two stages: a "priming" dose followed by a "provocative" dose, leading to microvascular thrombosis and hemorrhagic necrosis, typically in the kidneys (cortical necrosis). **Why Waterhouse-Friderichsen Syndrome (WFS) is correct:** WFS is characterized by massive, bilateral adrenal hemorrhage and acute adrenal insufficiency, most commonly complicating **Meningococcemia** (*Neisseria meningitidis*). The underlying pathophysiology involves widespread DIC and hemorrhagic necrosis triggered by endotoxemia, which is considered a clinical manifestation of the **generalized Shwartzman reaction**. **Analysis of Incorrect Options:** * **Fitz-Hugh-Curtis Syndrome:** This is perihepatitis (inflammation of the liver capsule) occurring as a complication of Pelvic Inflammatory Disease (PID), typically caused by *Chlamydia trachomatis* or *Neisseria gonorrhoeae*. It involves "violin-string" adhesions rather than Shwartzman-like necrosis. * **Eichwald-Silberberg Phenomenon:** Also known as the "H-Y effect," this refers to the immunological rejection of male skin grafts by female recipients of the same inbred strain due to male-specific antigens. * **Anaphylactoid Reaction:** This is a non-IgE mediated mast cell degranulation (e.g., due to radiocontrast media). Unlike the Shwartzman reaction, it is not dependent on endotoxins or priming/provocative dosing. **High-Yield Clinical Pearls for NEET-PG:** * **Shwartzman Reaction Key Mediator:** Interleukin-1 (IL-1) and TNF-alpha. * **Local Shwartzman:** Occurs at the site of injection (skin necrosis). * **Generalized Shwartzman:** Leads to **Bilateral Renal Cortical Necrosis**. * **WFS Triad:** Petechial rash, DIC, and Adrenal insufficiency. It is a medical emergency requiring immediate IV antibiotics and steroid replacement.

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