Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 121: A 40-year-old male patient complains of difficulty in breathing and has a history of multiple allergies to food and dust. Which immunoglobulin is characteristically elevated in this patient?

A. Immunoglobulin A (IgA)
B. Immunoglobulin M (IgM)
C. Immunoglobulin G (IgG)
D. Immunoglobulin E (IgE) (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Immunoglobulin E (IgE)** **Reasoning:** The patient presents with symptoms suggestive of **Type I Hypersensitivity (Atopy)**, characterized by a history of multiple allergies and respiratory distress (likely allergic asthma). In Type I hypersensitivity, initial exposure to an allergen leads to the production of **IgE antibodies** by plasma cells. These IgE molecules bind to high-affinity receptors (FcεRI) on the surface of **mast cells and basophils**. Upon re-exposure, the allergen crosses-links the bound IgE, triggering degranulation and the release of inflammatory mediators like histamine, leukotrienes, and prostaglandins, which cause bronchoconstriction and allergic symptoms. **Why other options are incorrect:** * **IgA:** Primarily involved in **mucosal immunity**. It is the most produced antibody in the body and is found in secretions like colostrum, saliva, and tears. It protects against pathogens at mucosal surfaces but is not the primary mediator of allergy. * **IgM:** The **first antibody produced** in a primary immune response. It exists as a pentamer and is effective at agglutination and complement activation. It indicates acute infection rather than chronic allergy. * **IgG:** The most abundant immunoglobulin in the serum and the only one that **crosses the placenta**. It is involved in secondary immune responses (anamnestic response) and Type II/III hypersensitivity, but not typically the acute allergic triggers seen here. **High-Yield Clinical Pearls for NEET-PG:** * **IgE** is the least abundant Ig in serum and is heat-labile (destroyed at 56°C for 30 mins). * **Prausnitz-Küstner (PK) reaction** was historically used to demonstrate IgE-mediated sensitivity. * IgE also plays a crucial role in **Type II antibody-dependent cellular cytotoxicity (ADCC)** against helminthic parasites (eosinophils bind to IgE-coated helminths). * **Hyper-IgE Syndrome (Job Syndrome):** Characterized by high IgE, recurrent "cold" staphylococcal abscesses, and eczema.

Question 122: Neonatal thymectomy leads to which of the following changes?

A. Decreased size of germinal centers
B. Decreased size of paracortical areas (Correct Answer)
C. Increased antibody production by B cells
D. Increased bone marrow production of lymphocytes

Explanation: **Explanation:** The thymus is the primary lymphoid organ responsible for the maturation and differentiation of **T-lymphocytes**. In the lymph nodes, T-cells are specifically localized in the **paracortical area** (the zone between the cortex and the medulla). **1. Why Option B is Correct:** Neonatal thymectomy removes the source of mature T-cells before they can seed peripheral lymphoid organs. Since the paracortex is the **T-cell dependent zone** of the lymph node, a lack of T-cells leads to the failure of this area to develop, resulting in its depletion or decreased size. Similarly, in the spleen, the periarteriolar lymphoid sheaths (PALS) would be depleted. **2. Analysis of Incorrect Options:** * **Option A:** Germinal centers are located in the lymphoid follicles of the cortex and are **B-cell dependent zones**. While T-helper cells are needed for isotype switching, the primary structure of the germinal center is maintained by B-cells and follicular dendritic cells. * **Option C:** T-cells (specifically T-helper cells) are required for most B-cell antibody responses (T-dependent antigens). Therefore, thymectomy would lead to **decreased**, not increased, antibody production. * **Option D:** The bone marrow is the site of lymphopoiesis (production). Thymectomy affects the maturation of T-cells, but it does not trigger a compensatory increase in the production of lymphoid progenitors in the marrow. **High-Yield Clinical Pearls for NEET-PG:** * **DiGeorge Syndrome:** A clinical "human model" of thymic aplasia (failure of 3rd and 4th pharyngeal pouches) characterized by T-cell deficiency and absent paracortical areas. * **T-cell Dependent Areas:** Paracortex of lymph nodes, PALS of the spleen. * **B-cell Dependent Areas:** Germinal centers, lymphoid follicles (cortex) of lymph nodes, and the white pulp (follicles) of the spleen. * **Nude Mice:** Laboratory mice born without a thymus, used frequently in immunology research to study T-cell deficiency.

Question 123: Immunoglobulin is used for all of the following diseases except?

A. Measles
B. Rabies
C. Typhoid (Correct Answer)
D. Chickenpox

Explanation: **Explanation:** The correct answer is **Typhoid (Option C)**. Immunoglobulin therapy involves **Passive Immunization**, where pre-formed antibodies are administered to provide immediate, short-term protection. This is typically used for post-exposure prophylaxis or in patients who cannot mount an immune response. **Why Typhoid is the correct answer:** Typhoid fever is caused by *Salmonella typhi*. Management and prevention rely on **Active Immunization** (e.g., Injectable Vi antigen or Oral Ty21a vaccine) and antibiotics for treatment. There is no clinical indication for the use of human or animal-derived immunoglobulins in the management of Typhoid. **Analysis of other options:** * **Measles:** Human Normal Immunoglobulin (HNIG) is used for post-exposure prophylaxis in susceptible individuals (especially infants and pregnant women) if given within 6 days of exposure. * **Rabies:** Rabies Immunoglobulin (RIG) is a critical component of Category III bite management, providing immediate neutralizing antibodies at the wound site before the vaccine-induced antibodies appear. * **Chickenpox:** Varicella-Zoster Immunoglobulin (VZIG) is indicated for high-risk individuals (e.g., immunocompromised or neonates) exposed to the virus to prevent or attenuate the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Passive-Active Immunity:** In Rabies and Tetanus, both immunoglobulin (passive) and vaccine (active) are given simultaneously at different sites. * **Types of Immunoglobulins:** * *Specific (Hyperimmune):* Rabies (RIG), Hepatitis B (HBIG), Varicella (VZIG), Tetanus (TIG). * *Non-specific:* Human Normal Immunoglobulin (used for Measles and Hepatitis A). * **Contraindication:** Immunoglobulins should generally not be administered within 3 months of live attenuated vaccines (like MMR or Varicella) as they may interfere with the immune response.

Question 124: Real-time PCR is used for what purpose?

A. Multiplication of RNA
B. Multiplication of Proteins
C. Multiplication of specific segments of DNA
D. Quantitative detection of PCR material (Correct Answer)

Explanation: **Explanation:** **Real-time PCR (qPCR)** is a modification of the standard PCR technique that allows for the **simultaneous amplification and quantification** of a specific DNA sequence. Unlike conventional PCR, which uses "end-point analysis" (detecting DNA after the reaction is finished via gel electrophoresis), Real-time PCR uses fluorescent dyes or probes to monitor the accumulation of amplicons **during** the exponential phase of the reaction. The intensity of the fluorescence is directly proportional to the amount of DNA template present, making it the gold standard for **quantitative detection**. **Analysis of Options:** * **Option A (Multiplication of RNA):** PCR specifically amplifies DNA. To amplify RNA, it must first be converted to cDNA via **Reverse Transcription (RT-PCR)**. * **Option B (Multiplication of Proteins):** PCR does not involve proteins; protein detection is typically done via Western Blot or ELISA. * **Option C (Multiplication of specific segments of DNA):** While this is the basic function of *conventional* PCR, the specific advantage and primary purpose of "Real-time" PCR is the **quantification** of that material. **High-Yield Clinical Pearls for NEET-PG:** * **Ct Value (Cycle Threshold):** In qPCR, the Ct value is inversely proportional to the viral load. A **lower Ct value** indicates a **higher viral load** (detected earlier). * **Clinical Use:** It is the preferred method for monitoring **Viral Load** in HIV, Hepatitis B/C, and was the diagnostic mainstay for **SARS-CoV-2**. * **Fluorescent Markers:** Commonly uses non-specific dyes like **SYBR Green** or sequence-specific probes like **TaqMan**. * **Advantage:** It is faster, more sensitive, and reduces the risk of carry-over contamination as the tube remains closed.

Question 125: Precipitation and agglutination are primarily caused by which immunoglobulins?

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

Explanation: **Explanation:** The reaction between a soluble antigen and its antibody resulting in an insoluble visible complex is **Precipitation**, while the clumping of particulate antigens (like bacteria or RBCs) by antibodies is **Agglutination**. **Why IgG and IgM are correct:** * **IgM (The Best Agglutinator):** Due to its pentameric structure, IgM has 10 antigen-binding sites (high valency). This allows it to bridge multiple particulate antigens effectively, making it the most potent immunoglobulin for agglutination (e.g., ABO blood grouping). * **IgG (The Best Precipitator):** Although bivalent, IgG is highly effective at lattice formation with soluble antigens. In many clinical assays (like VDRL or radial immunodiffusion), IgG plays a primary role in forming the visible precipitate. **Analysis of Incorrect Options:** * **IgA:** Primarily a secretory antibody found in colostrum, saliva, and mucosal surfaces. Its main role is mucosal immunity (neutralization), not systemic precipitation or agglutination. * **IgD:** Functions mainly as a surface receptor on B-cells; it is present in negligible amounts in serum and does not participate in these reactions. * **IgE:** Involved in Type I Hypersensitivity and parasitic infections; it triggers mast cell degranulation rather than forming lattices. **High-Yield Clinical Pearls for NEET-PG:** 1. **Prozone Phenomenon:** False negative agglutination due to **antibody excess**. 2. **Postzone Phenomenon:** False negative due to **antigen excess**. 3. **Coombs Test:** Uses "Antihuman Globulin" to induce agglutination when IgG (an incomplete antibody) is unable to bridge the distance between RBCs on its own. 4. **Valency:** IgM has a theoretical valency of 10, but an effective valency of 5 due to steric hindrance.

Question 126: Which of the following statements is true about Interferon-?

A. It is virus specific
B. It is Bacteria specific
C. Produced from Bacteria
D. Effective against viral infection (Correct Answer)

Explanation: **Explanation:** Interferons (IFNs) are a group of signaling proteins (cytokines) released by host cells in response to the presence of several pathogens, most notably viruses. **Why the correct answer is right:** **Option D (Effective against viral infection):** Interferons do not kill viruses directly. Instead, they act as "warning signals." When a cell is infected by a virus, it releases IFNs which bind to receptors on neighboring uninfected cells. This triggers the production of **antiviral proteins (AVPs)**, such as *Oligoadenylate synthetase* and *Protein kinase R*, which inhibit viral protein synthesis and degrade viral RNA, thereby limiting the spread of the infection. **Why the incorrect options are wrong:** * **Option A (Virus specific):** Interferons are **not virus-specific**. An interferon produced in response to one virus (e.g., Influenza) will provide protection against a wide range of other unrelated viruses (e.g., Hepatitis). * **Option B (Bacteria specific):** While some IFNs (like IFN-gamma) play a role in activating macrophages to fight bacteria, their primary and most characteristic diagnostic role is in the non-specific antiviral response. * **Option C (Produced from Bacteria):** Interferons are produced by **host (mammalian) cells** (e.g., leukocytes, fibroblasts, and T-cells), not by bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Species Specificity:** IFNs are **species-specific**. Human interferons work only on human cells, which is why recombinant human IFNs are used for therapy. * **Types of IFNs:** * **Type I (IFN-α, IFN-β):** Produced by leukocytes and fibroblasts; primary antiviral role. * **Type II (IFN-γ):** Produced by Th1 cells and NK cells; primary role is immunomodulation and macrophage activation. * **Clinical Use:** IFN-α is used in the treatment of Hepatitis B, Hepatitis C, and Kaposi Sarcoma.

Question 127: What is the most common cause of urinary tract infection in neonates?

A. E. coli (Correct Answer)
B. S. aureus
C. Enterococcus
D. Anaerobes

Explanation: **Explanation:** **Correct Option: A. E. coli** In neonates, urinary tract infections (UTIs) are primarily caused by organisms originating from the maternal fecal flora or the infant's own gastrointestinal tract. **Escherichia coli** is the most common pathogen across all age groups, including neonates, accounting for approximately 80% of cases. In the neonatal period, UTIs are more common in males (especially uncircumcised) and are often associated with **hematogenous spread** (bacteremia) rather than the ascending route typical in older children. **Analysis of Incorrect Options:** * **B. S. aureus:** While *S. aureus* can cause UTIs via the hematogenous route during neonatal sepsis, it is significantly less common than Gram-negative bacilli. * **C. Enterococcus:** *Enterococcus species* (Group D Streptococci) are known causes of neonatal UTI and sepsis, but they rank second or third behind *E. coli* and *Klebsiella*. * **D. Anaerobes:** Anaerobic bacteria are extremely rare causes of UTI. They are typically only considered in cases of structural abnormalities or chronic obstructive uropathy. **High-Yield Clinical Pearls for NEET-PG:** * **Gender Predominance:** In the neonatal period, UTIs are more common in **males** (M:F ratio ~3:1). After the first year of life, the incidence shifts significantly toward females. * **Route of Infection:** In neonates, the primary route is **hematogenous**; in older children, it is **ascending**. * **Associated Findings:** Any neonate with a UTI must be evaluated for **Vesicoureteral Reflux (VUR)** or structural anomalies like Posterior Urethral Valves (PUV) via ultrasound and MCUG (Micturating Cystourethrogram). * **Other Pathogens:** After *E. coli*, the next most common organisms are *Klebsiella pneumoniae*, *Enterococcus*, and *Proteus*.

Question 128: Transplacental transfer of antibodies from an immunized mother to her newborn provides which type of immunity?

A. Natural active immunity
B. Natural passive immunity (Correct Answer)
C. Acquired active immunity
D. Acquired passive immunity

Explanation: ### Explanation **Correct Answer: B. Natural passive immunity** The transfer of maternal antibodies (IgG) across the placenta to the fetus is a classic example of **Natural Passive Immunity**. 1. **Why it is "Passive":** The newborn’s immune system is not actively involved in producing these antibodies. Instead, pre-formed antibodies are "passed" from the mother to the fetus. 2. **Why it is "Natural":** This process occurs through a biological, physiological mechanism (pregnancy) rather than a medical intervention like an injection. --- ### Analysis of Incorrect Options: * **A. Natural active immunity:** This occurs when a person is naturally exposed to a live pathogen (e.g., catching Chickenpox), and their own immune system produces antibodies and memory cells. * **C. Acquired active immunity:** This is induced by **vaccination**. The body is deliberately exposed to an antigen (e.g., Hepatitis B vaccine) to stimulate its own immune response. * **D. Acquired passive immunity:** This involves the administration of pre-formed antibodies via medical intervention, such as **Ready-made Antisera** (e.g., Anti-tetanus serum or Rabies immunoglobulin) to provide immediate protection. --- ### NEET-PG High-Yield Pearls: * **IgG** is the **only** immunoglobulin class that can cross the placenta, providing the newborn with protection for the first few months of life. * **Colostrum** (breast milk) provides **Natural Passive Immunity** via **Secretory IgA**. * **Passive immunity** provides immediate protection but is **short-lived** because no memory cells are formed and the transferred antibodies eventually degrade. * **Active immunity** has a "lag period" before becoming effective but provides **long-lasting** protection due to the formation of memory B and T cells.

Question 129: During the infancy of cardiac transplantation, nonhuman primate hearts were transplanted into humans to save lives. This type of transplantation is best described by which one of the following?

A. Allograft: transplant from one individual to another of the same species
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 (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Xenograft**. **1. Understanding the Correct Answer:** A **Xenograft** (or heterograft) refers to the transplantation of cells, tissues, or organs between members of **different species**. In the scenario described, the transfer of a heart from a nonhuman primate (e.g., baboon or chimpanzee) to a human represents a cross-species barrier. These grafts are highly immunogenic and typically trigger **hyperacute rejection** due to pre-existing natural antibodies (e.g., anti-Gal antibodies) against foreign surface antigens. **2. Analysis of Incorrect Options:** * **A. Allograft:** This is a transplant between genetically non-identical members of the **same species** (e.g., human to human). This is the most common type of clinical transplant. * **B. Autograft:** This involves moving tissue from one site to another on the **same individual** (e.g., a saphenous vein graft for CABG or a skin graft). There is no risk of immunological rejection. * **C. Isograft (Syngeneic graft):** This is a transplant between **genetically identical** individuals, such as monozygotic (identical) twins. Like autografts, these do not trigger an immune response. **3. NEET-PG Clinical Pearls:** * **Hyperacute Rejection:** Occurs within minutes to hours; mediated by **pre-formed antibodies** and Type II hypersensitivity. It is the primary barrier in Xenotransplantation. * **Acute Rejection:** Occurs within days to weeks; primarily mediated by **T-cells** (Type IV hypersensitivity). * **Chronic Rejection:** Occurs months to years later; characterized by fibrosis and **accelerated arteriosclerosis** (intimal thickening). * **Graft-versus-Host Disease (GVHD):** Occurs when immunocompetent T-cells in the graft attack the immunocompromised host (common in bone marrow transplants).

Question 130: Antigen-antibody precipitation is maximally observed under which of the following conditions?

A. Excess of antibody
B. Excess of antigen
C. Equivalence of antibody and antigen (Correct Answer)
D. Antigen-hapten interaction

Explanation: ### Explanation The phenomenon of antigen-antibody precipitation is best explained by Marrack’s **Lattice Hypothesis**. For a visible precipitate to form, multivalent antigens and bivalent antibodies must cross-link to create a large, insoluble three-dimensional network (lattice). **1. Why Option C is Correct:** Precipitation is maximal at the **Zone of Equivalence**. In this zone, the ratio of antigen to antibody is optimal, allowing every antigen molecule to be cross-linked by antibodies into a large, heavy aggregate that settles out of the solution. **2. Why Incorrect Options are Wrong:** * **Option A (Prozone Phenomenon):** In the zone of **antibody excess**, each antigen molecule is rapidly saturated by multiple antibodies, preventing the cross-linking required to form a large lattice. This results in small, soluble complexes. * **Option B (Postzone Phenomenon):** In the zone of **antigen excess**, every antibody binding site is quickly occupied by a separate antigen molecule. There are insufficient antibodies to bridge the antigens together, leading to small, soluble complexes. * **Option D:** Haptens are univalent (possess only one epitope). While they can bind to antibodies, they cannot form a lattice because they cannot bridge two different antibody molecules. **Clinical Pearls for NEET-PG:** * **Prozone Phenomenon** can lead to **false-negative** results in serological tests (e.g., VDRL for Syphilis or 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, whereas agglutination involves **particulate/insoluble** antigens (like RBCs or bacteria). * **Immunodiffusion:** Tests like the **Elek’s test** (for *C. diphtheriae* toxin) rely on the principle of precipitation at the zone of equivalence.

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Cells and Organs of Immune System

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Antigens and Antibodies

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Complement System

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Immunology — MCQs

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