Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 151: All of the following statements about carbohydrate antigen are true except?

A. It has lower immunogenicity.
B. Memory response is seen. (Correct Answer)
C. Causes polyclonal B cell stimulation.
D. Does not require stimulation by T cells.

Explanation: ### Explanation This question tests the fundamental distinction between **T-dependent (TD)** and **T-independent (TI)** antigens. Carbohydrate antigens (like bacterial capsular polysaccharides) are classic examples of **T-independent antigens**. **1. Why "Memory response is seen" is the Correct Answer (The False Statement):** T-independent antigens (carbohydrates) interact directly with B cells without the help of T-helper (CD4+) cells. Because T-cell involvement is absent, there is no **isotype switching** (from IgM to IgG) and no formation of **memory B cells**. Consequently, every exposure to a carbohydrate antigen results in a primary-like immune response (predominantly IgM) with no immunological memory. **2. Analysis of Incorrect Options:** * **A. Lower immunogenicity:** True. Carbohydrates are generally less immunogenic than proteins because they are structurally repetitive and cannot be presented via MHC-II molecules to T cells. * **C. Polyclonal B cell stimulation:** True. Many TI antigens (specifically TI-Type 1, like LPS) act as mitogens, capable of activating multiple B cell clones regardless of their antigen specificity. * **D. Does not require stimulation by T cells:** True. This is the defining feature of TI antigens. They possess repetitive epitopes that cross-link B-cell receptors (BCR) directly, triggering activation without T-cell "help." ### High-Yield Clinical Pearls for NEET-PG: * **Conjugate Vaccines:** To induce memory against carbohydrate antigens (e.g., *H. influenzae* type b or Pneumococcal vaccines), the polysaccharide is conjugated to a **protein carrier**. This converts the TI antigen into a TD antigen, allowing T-cell involvement and long-term memory. * **Age Factor:** Children under 2 years of age have poorly developed TI responses, which is why they are highly susceptible to encapsulated bacteria (e.g., *S. pneumoniae*, *N. meningitidis*). * **Hapten vs. Antigen:** A hapten is a small molecule that is antigenic but not immunogenic unless attached to a protein carrier.

Question 152: The nitroblue tetrazolium (NBT) reduction assay is used to determine what?

A. Granulocyte function
B. T-cell function
C. Polymorphonuclear leucocytes' ability to produce superoxide (Correct Answer)
D. B-lymphocyte staining

Explanation: **Explanation:** The **Nitroblue Tetrazolium (NBT) reduction assay** is a classic screening test used to evaluate the metabolic activity of phagocytes, specifically their ability to undergo a "respiratory burst." **Why Option C is Correct:** During phagocytosis, polymorphonuclear leukocytes (neutrophils) activate the **NADPH oxidase enzyme complex** to produce **superoxide radicals**. In the NBT test, the colorless, water-soluble NBT dye is added to the patient's neutrophils. If the cells are functioning correctly and producing superoxide, the NBT is reduced into an insoluble, dark blue-black substance called **formazan**. A positive result (blue color) indicates intact oxidative killing power. **Why Other Options are Incorrect:** * **Option A:** While it technically tests a "function" of granulocytes, "Granulocyte function" is too broad. It could refer to chemotaxis or adhesion; the NBT test specifically measures the **oxidative burst**. * **Option B:** T-cell function is typically assessed via delayed-type hypersensitivity (DTH) skin tests or flow cytometry (CD3/CD4/CD8 counts), not oxidative assays. * **Option D:** B-lymphocyte staining involves identifying surface markers like CD19/CD20 or surface immunoglobulins, unrelated to superoxide production. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** This is the primary condition diagnosed using the NBT test. In CGD, there is a genetic defect in NADPH oxidase; therefore, neutrophils **fail to reduce NBT**, and the dye remains colorless. * **Modern Alternative:** The **Dihydrorhodamine (DHR) flow cytometry assay** is now the preferred gold standard over NBT due to higher sensitivity and quantitative results. * **Catalase-positive organisms:** Patients with CGD are specifically susceptible to infections by *Staphylococcus aureus*, *Aspergillus*, and *Serratia marcescens*.

Question 153: In C-reactive protein, the 'C' stands for what?

A. Capsular polysaccharide in pneumococcus (Correct Answer)
B. Concanavalin-A
C. Calretinin
D. Cellular

Explanation: **Explanation:** **C-reactive protein (CRP)** is an acute-phase reactant synthesized by the liver in response to IL-6. It was first discovered by Tillett and Francis in 1930 in the sera of patients with pneumonia. It was named "C-reactive" because it has the unique property of precipitating with the **'C' capsular polysaccharide of *Streptococcus pneumoniae*** (Pneumococcus). * **Why Option A is correct:** CRP binds to phosphocholine expressed on the surface of the pneumococcal C-polysaccharide. This binding activates the classical complement pathway, promoting opsonization and phagocytosis of the pathogen. * **Why Option B is incorrect:** **Concanavalin-A** is a lectin derived from jack beans; while it is used in immunology as a T-cell mitogen, it has no structural or nomenclature relationship with CRP. * **Why Option C is incorrect:** **Calretinin** is a calcium-binding protein used primarily as an immunohistochemical marker for diagnosing mesothelioma. * **Why Option D is incorrect:** While CRP is part of the innate immune response, the 'C' is a specific biochemical reference to the bacterial ligand, not a general "cellular" description. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** CRP is a member of the **pentraxin** family (five identical non-covalently linked subunits). * **Function:** It acts as an **opsonin** and is a sensitive marker of systemic inflammation. * **Kinetics:** Levels rise rapidly (within 6–12 hours) and have a short half-life (19 hours), making it an excellent marker for monitoring disease activity and response to antibiotics. * **hs-CRP:** High-sensitivity CRP is used as a predictive marker for **cardiovascular risk** (atherosclerosis is an inflammatory process).

Question 154: What is the type of antigen-antibody reaction seen in VDRL?

A. Agglutination
B. Flocculation (Correct Answer)
C. Passive agglutination
D. Gel precipitation

Explanation: ### Explanation **Correct Answer: B. Flocculation** **Why it is correct:** The VDRL (Venereal Disease Research Laboratory) test is a non-specific screening test for Syphilis. It utilizes a **flocculation** reaction, which is a specific type of precipitation. In this reaction, the soluble antigen (Cardiolipin) reacts with the antibody (Reagin) in the patient's serum. Instead of forming a sediment at the bottom (as in standard precipitation), the antigen-antibody complexes remain suspended as visible **flakes or clumps** (floccules) due to the lipids involved. This is typically visualized under a light microscope. **Why the other options are incorrect:** * **A. Agglutination:** This involves the clumping of **particulate** antigens (like whole bacteria or RBCs). In VDRL, the antigen is soluble cardiolipin, not a particle. * **C. Passive Agglutination:** This occurs when a soluble antigen is artificially coated onto a carrier particle (like latex or charcoal) to convert a precipitation reaction into an agglutination reaction. An example is the **RPR (Rapid Plasma Reagin)** test, which uses charcoal particles. * **D. Gel Precipitation:** This involves the diffusion of antigens and antibodies through an agar gel (e.g., Elek’s test). VDRL is performed in a liquid medium (on a slide). **High-Yield Clinical Pearls for NEET-PG:** * **Antigen used in VDRL:** Cardiolipin (derived from beef heart), cholesterol, and lecithin. * **RPR vs. VDRL:** RPR is a modified flocculation test that uses charcoal particles, allowing the results to be read with the naked eye (Passive Agglutination). VDRL requires a microscope. * **Prozone Phenomenon:** A false-negative VDRL result can occur due to very high antibody titers (seen in secondary syphilis). * **Biological False Positives (BFP):** VDRL can be positive in non-syphilitic conditions like SLE, Leprosy, Malaria, and Pregnancy.

Question 155: Where is the HLA antigen located?

A. Short arm of chromosome 6 (Correct Answer)
B. Long arm of chromosome 6
C. Short arm of chromosome 8
D. Long arm of chromosome 8

Explanation: **Explanation:** The **Human Leukocyte Antigen (HLA)** system is the human version of the **Major Histocompatibility Complex (MHC)**. These genes are located on the **short arm (p arm)** of **Chromosome 6**. **1. Why Option A is Correct:** The HLA complex is a large cluster of genes located at position **6p21.3**. It is divided into three classes: * **Class I (A, B, C):** Encodes proteins found on all nucleated cells. * **Class II (DR, DQ, DP):** Encodes proteins found on antigen-presenting cells (APCs). * **Class III:** Encodes components of the complement system (C2, C4) and cytokines like TNF. The proximity of these genes on the short arm of chromosome 6 ensures they are usually inherited together as a **haplotype**. **2. Why Other Options are Incorrect:** * **Option B:** The long arm (q arm) of chromosome 6 contains other important genes (like those for certain enzyme deficiencies), but not the MHC complex. * **Options C & D:** Chromosome 8 is associated with different medical conditions, most notably the **c-myc oncogene** (8q24), which is involved in Burkitt’s Lymphoma. It has no role in HLA encoding. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** HLA genes are **codominantly** expressed. * **Beta-2 Microglobulin:** While the HLA heavy chain is on chromosome 6, the $\beta$2-microglobulin component of Class I MHC is encoded on **Chromosome 15**. * **Disease Associations:** * **HLA-B27:** Ankylosing spondylitis. * **HLA-DR3/DR4:** Type 1 Diabetes Mellitus. * **HLA-DQ2/DQ8:** Celiac disease. * **HLA-B51:** Behcet’s disease.

Question 156: Synthesis of an immunoglobulin in membrane-bound or secretory form is determined by which mechanism?

A. One turn to two turn joining rule
B. Class switching
C. Differential RNA processing (Correct Answer)
D. Allelic exclusion

Explanation: ### Explanation The correct answer is **Differential RNA processing** (also known as alternative splicing). #### 1. Why Differential RNA Processing is Correct Every B-cell must decide whether to produce an immunoglobulin (Ig) that stays anchored to its surface (B-cell receptor) or one that is secreted into the plasma (antibody). This is determined **after transcription** but **before translation**. * The heavy chain gene contains two distinct polyadenylation (poly-A) sites at its 3' end: one for the **membrane-bound** form (containing a hydrophobic transmembrane segment) and one for the **secretory** form. * By choosing which site to use during RNA splicing, the cell determines the final destination of the protein without changing the DNA sequence or the antigen specificity. #### 2. Why Other Options are Incorrect * **A. One turn to two turn joining rule (12/23 rule):** This ensures that V, D, and J gene segments are joined in the correct order during V(D)J recombination. It relates to diversity, not the form of the Ig. * **B. Class switching (Isotype switching):** This is a **DNA-level** rearrangement that changes the constant region (e.g., from IgM to IgG). It changes the *function* of the antibody but does not determine if it is membrane-bound or secreted. * **D. Allelic exclusion:** This process ensures that a B-cell expresses an immunoglobulin from only one of its two parental alleles, ensuring **monospecificity**. #### 3. NEET-PG High-Yield Pearls * **DNA Level Changes:** V(D)J Recombination and Class Switching (Irreversible). * **RNA Level Changes:** Synthesis of membrane vs. secretory forms and the simultaneous expression of **IgM and IgD** on mature B-cells (both use differential splicing). * **Secretory Component:** While RNA processing determines if an Ig is secreted, the **J-chain** is specifically required for the dimerization of IgA and pentamerization of IgM.

Question 157: A 10-year-old male presented to the pediatric OPD with colicky pain, nausea, and vomiting. An erect abdominal X-ray shows multiple air-fluid levels. Stool examination shows a specific type of egg. Which of the following interleukin is secreted in this scenario?

A. IL-1
B. IL-2
C. IL-3
D. IL-5 (Correct Answer)

Explanation: ### Explanation **Clinical Correlation:** The clinical presentation of colicky pain, vomiting, and air-fluid levels on X-ray indicates **intestinal obstruction**. In a pediatric patient, a common parasitic cause of obstruction is *Ascaris lumbricoides* (roundworm). The mention of "specific eggs" in the stool confirms a **Helminthic infection**. **Why IL-5 is Correct:** Helminthic infections trigger a **Type 2 Helper T-cell (Th2) response**. Th2 cells secrete a specific cytokine profile (IL-4, IL-5, and IL-13) to combat extracellular parasites: * **IL-5** is the primary cytokine responsible for the **recruitment, activation, and survival of eosinophils**. Eosinophils release major basic protein (MBP) to damage the parasite's tegument. * **IL-4 and IL-13** stimulate B-cell class switching to **IgE**, which coats the parasite and facilitates antibody-dependent cellular cytotoxicity (ADCC). **Why Other Options are Incorrect:** * **IL-1:** A pro-inflammatory cytokine produced by macrophages. It mediates the acute phase response and fever; it is not specific to the Th2 anti-parasitic response. * **IL-2:** Produced by Th1 cells; it acts as a T-cell growth factor and is essential for the proliferation of CD4+ and CD8+ T-cells. * **IL-3:** A multilineage colony-stimulating factor that supports the growth of hematopoietic stem cells in the bone marrow. While it can support mast cell growth, it is not the primary mediator for eosinophilia in helminthiasis. **NEET-PG High-Yield Pearls:** * **Th1 Response:** Secretes IFN-γ, IL-2, and TNF-β (targets intracellular pathogens like *M. tuberculosis*). * **Th2 Response:** Secretes IL-4, IL-5, IL-10, and IL-13 (targets extracellular parasites/allergens). * **Eosinophilia + IgE elevation** is the classic laboratory hallmark of helminthic infestations. * *Ascaris lumbricoides* is the most common cause of parasitic intestinal obstruction worldwide.

Question 158: A 21-year-old patient in severe kidney failure receives a kidney from his 30-year-old brother. This type of transplantation is best described by which of the following?

A. Allograft: transplant from one individual to another of the same species (Correct Answer)
B. Autograft: transplant from one region of a person to another region
C. Isograft: transplant from one person to a genetically identical person
D. Xenograft: transplant from one species to another species

Explanation: ### Explanation The correct answer is **A. Allograft**. In clinical transplantation, the classification of grafts is based on the genetic relationship between the donor and the recipient. **1. Why Allograft is correct:** An **Allograft** (or homograft) is a transplant between two genetically different individuals of the same species. Since the patient and his brother are siblings, they share some genetic material but are not genetically identical (unless they are monozygotic twins). This is the most common type of organ transplantation performed in clinical practice. **2. Analysis of Incorrect Options:** * **B. Autograft:** This involves moving tissue from one site to another on the **same individual** (e.g., a skin graft from the thigh to the arm or a CABG using the saphenous vein). There is no risk of rejection. * **C. Isograft (Syngeneic graft):** This is a transplant between **genetically identical** individuals, such as monozygotic (identical) twins. Because the MHC molecules are identical, there is no immune response or rejection. * **D. Xenograft:** This is a transplant between members of **different species** (e.g., a porcine/pig heart valve transplanted into a human). These are subject to rapid hyperacute rejection due to pre-existing antibodies. **3. NEET-PG High-Yield Pearls:** * **MHC/HLA Matching:** The success of an allograft depends primarily on the matching of Human Leukocyte Antigens (HLA), specifically **HLA-DR, HLA-B, and HLA-A** (in that order of importance for kidney transplants). * **Rejection Types:** * **Hyperacute:** Minutes to hours (Pre-formed antibodies). * **Acute:** Days to weeks (T-cell mediated/Cellular). * **Chronic:** Months to years (Fibrosis and vascular thickening). * **Graft-versus-Host Disease (GVHD):** Most common in bone marrow transplants where the donor's T-cells attack the recipient's tissues.

Question 159: What is the primary complex of tuberculosis known as?

A. Ranke's complex
B. Ghon's complex (Correct Answer)
C. Assman focus
D. Simon's focus

Explanation: **Explanation:** The **Ghon’s complex** (Option B) is the hallmark of primary tuberculosis. It represents the initial site of infection in a non-immune host (usually a child). It consists of three components: 1. **Ghon Focus:** A small parenchymal granuloma, typically located in the subpleural region of the upper part of the lower lobe or lower part of the upper lobe. 2. **Lymphangitis:** Inflammation of the lymphatic vessels draining the focus. 3. **Hilar Lymphadenopathy:** Involvement of the draining paratracheal or hilar lymph nodes. **Analysis of Incorrect Options:** * **Ranke’s Complex (Option A):** This is the late, healed stage of a Ghon’s complex that has undergone fibrosis and **calcification**, visible on a chest X-ray. * **Assman Focus (Option C):** This refers to an area of infra-clavicular infiltration seen in **secondary (reactivation) tuberculosis**, rather than primary infection. * **Simon’s Focus (Option D):** These are apical nodules formed due to hematogenous seeding during primary infection. They often heal but can reactivate later in life to cause secondary TB. **High-Yield Clinical Pearls for NEET-PG:** * **Puhl’s Focus:** A specific term for the initial lesion of secondary TB in the lung apex. * **Rich Focus:** A subpial or subependymal tubercle in the brain that ruptures into the subarachnoid space, leading to TB meningitis. * **Primary TB** is characterized by the Ghon complex, while **Secondary TB** is characterized by cavitation and apical localization.

Question 160: Active and passive immunity should be given together in all except which of the following conditions?

A. Tetanus
B. Rabies
C. Measles (Correct Answer)
D. Hepatitis B

Explanation: **Explanation:** The simultaneous administration of active and passive immunity (simultaneous immunization) is indicated in conditions with a high fatality rate or high risk of transmission where immediate protection is required while the body develops its own long-term immune response. **Why Measles is the Correct Answer:** In Measles, active and passive immunization are **never** given together. If a susceptible individual is exposed, they are given either the Measles vaccine (within 72 hours) **OR** Immunoglobulin (within 6 days). If Immunoglobulin is administered, it interferes with the replication of the live-attenuated measles virus in the vaccine, rendering the vaccine ineffective. Therefore, a gap of at least **3 to 11 months** (depending on the dose of Ig) is required before administering the measles vaccine. **Analysis of Incorrect Options:** * **Tetanus:** In "tetanus-prone" wounds, both Tetanus Toxoid (TT/Td) and Tetanus Immunoglobulin (TIG) are given at different sites to provide immediate and lasting coverage. * **Rabies:** Post-exposure prophylaxis (Category III bites) mandates the administration of the Rabies vaccine and Rabies Immunoglobulin (RIG) to neutralize the virus at the site before it enters the nervous system. * **Hepatitis B:** For needle-stick injuries in non-immune individuals or infants born to HBsAg-positive mothers, both the Hep B vaccine and HBIG are administered simultaneously. **High-Yield Clinical Pearls for NEET-PG:** * **Site Rule:** When giving active and passive immunity together, always use **separate syringes** and **separate anatomical sites** (e.g., different arms). * **Other examples** of simultaneous immunization include Diphtheria and Varicella (in specific post-exposure scenarios). * **Live Vaccines Rule:** Generally, live vaccines should be avoided for several months after receiving immunoglobulin/blood products, with the exception of Yellow Fever vaccine.

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Immunology — MCQs

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