Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 91: Which immunoglobulin is produced first by the fetus in response to infection?

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

Explanation: **Explanation:** The correct answer is **IgM**. **Why IgM is correct:** IgM is the first immunoglobulin class produced by the fetus during intrauterine life, starting around the 20th week of gestation. Unlike IgG, IgM is a large pentameric molecule that **cannot cross the placenta**. Therefore, if IgM antibodies are detected in the cord blood or neonatal serum, it is a definitive diagnostic indicator of an **intrauterine (congenital) infection** (e.g., TORCH infections), as these antibodies must have been synthesized by the fetus’s own immune system in response to a pathogen. **Why other options are incorrect:** * **IgG:** While IgG is the most abundant immunoglobulin in a neonate, it is almost entirely derived from the mother via **passive placental transfer**. The fetus only begins significant endogenous production of IgG after birth. * **IgA:** Secretory IgA is primarily involved in mucosal immunity and is provided to the infant through colostrum and breast milk. Fetal production of IgA is negligible. * **IgD:** This is primarily found on the surface of B-lymphocytes as a receptor and is not produced in significant quantities by the fetus in response to infection. **High-Yield Clinical Pearls for NEET-PG:** * **IgG:** The only antibody that crosses the placenta (provides passive immunity). * **IgM:** The first antibody produced in a primary immune response (both in adults and fetuses). * **IgA:** The most abundant antibody in body secretions (tears, saliva, breast milk). * **IgE:** Mediates Type I hypersensitivity and provides defense against helminthic infections. * **Memory Trick:** **M** is for **M**omentary/Immediate (first response) and **G** is for **G**estation (crosses placenta).

Question 92: At what zone do antigen-antibody reactions occur most effectively?

A. Prozone
B. Postzone
C. Zone of equivalence (Correct Answer)
D. All of the above

Explanation: **Explanation:** The antigen-antibody reaction is governed by the **Marrack’s Lattice Hypothesis**, which states that for a visible precipitate to form, multivalent antigens must be cross-linked by bivalent antibodies to create a large, insoluble structural network (lattice). **1. Why Zone of Equivalence is Correct:** The **Zone of Equivalence** is the point where the concentration of antigen and antibody is optimal (roughly equal). In this zone, every antigen binding site is effectively cross-linked by an antibody, leading to the maximum formation of large lattices. This results in the most rapid and visible precipitation or agglutination. **2. Why Other Options are Incorrect:** * **Prozone (Antibody Excess):** When antibodies are present in very high concentrations, each antigenic determinant is saturated by a single antibody molecule. This prevents the cross-linking required to form a lattice, resulting in a false-negative reaction. * **Postzone (Antigen Excess):** When antigens are in excess, every antibody binding site is quickly occupied by a single antigen. There are insufficient antibodies to bridge the antigens together, again failing to form a lattice and leading to a false-negative result. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **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, whereas agglutination involves **particulate/insoluble** antigens (e.g., RBCs or bacteria). * **Valency:** For lattice formation, the antigen must be multivalent and the antibody must be at least bivalent (IgG) or multivalent (IgM).

Question 93: Cytotoxic T cells induced by infection with virus A will kill target cells that are infected by virus A and identical at class I MHC loci to the cytotoxic T cells?

A. From the same host infected with any virus
B. Infected by virus A and identical at class I MHC loci to the cytotoxic T cells (Correct Answer)
C. Infected by virus A and identical at class II MHC loci to the cytotoxic T cells
D. Infected with any virus and identical at class I MHC loci to the cytotoxic cells

Explanation: ### Explanation The correct answer is **B: Infected by virus A and identical at class I MHC loci to the cytotoxic T cells.** #### 1. Why the Correct Answer is Right: MHC Restriction This question tests the concept of **MHC Restriction**, specifically for **CD8+ Cytotoxic T cells (CTLs)**. * **Antigen Specificity:** T-cell receptors (TCRs) are highly specific. CTLs induced by Virus A will only recognize and kill cells presenting peptides derived from Virus A. * **MHC Class I Restriction:** CD8+ T cells only recognize antigens presented on **MHC Class I** molecules (found on all nucleated cells). * **Self-Recognition:** For a CTL to kill a target, the MHC molecule must be "self" (syngeneic). This was famously demonstrated by Zinkernagel and Doherty, showing that T cells must recognize both the **foreign viral antigen** and the **self-MHC molecule** simultaneously. #### 2. Analysis of Incorrect Options * **Option A & D:** These are incorrect because they suggest "any virus." CTLs are antigen-specific; they will not kill cells infected by a different virus (e.g., Virus B) because the TCR will not recognize the peptide. * **Option C:** This is incorrect because **MHC Class II** molecules are recognized by **CD4+ Helper T cells**, not CD8+ Cytotoxic T cells. Class II molecules are primarily found on professional Antigen-Presenting Cells (APCs). #### 3. High-Yield Clinical Pearls for NEET-PG * **Rule of 8:** Remember the multiplication rule: **CD4 × MHC II = 8** and **CD8 × MHC I = 8**. * **Endogenous vs. Exogenous:** MHC Class I presents **endogenous** antigens (viruses, tumors), while MHC Class II presents **exogenous** antigens (extracellular bacteria). * **MHC Loci:** In humans, MHC Class I corresponds to **HLA-A, B, and C**; MHC Class II corresponds to **HLA-DP, DQ, and DR**. * **Transplant Immunology:** MHC restriction is the reason why HLA matching is critical in organ transplantation to prevent graft rejection.

Question 94: Which CD marker is characteristic of NK cells?

A. CD16 (Correct Answer)
B. CD60
C. CD32
D. CD25

Explanation: **Explanation:** **Natural Killer (NK) cells** are large granular lymphocytes that play a critical role in the innate immune system. The correct answer is **CD16** (also known as FcγRIII). This marker is a low-affinity receptor for the Fc portion of IgG antibodies. Its presence allows NK cells to bind to antibody-coated target cells and destroy them via **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. Along with **CD56** (NCAM), CD16 is considered a definitive phenotypic marker for identifying NK cells in clinical practice. **Analysis of Incorrect Options:** * **CD60:** This is a carbohydrate antigen (sialylated ganglioside) primarily found on a subset of T cells and melanocytes; it is not a primary marker for NK cells. * **CD32 (FcγRII):** This is an inhibitory Fc receptor found on B cells, macrophages, and neutrophils. It regulates immune responses but is not the characteristic marker for NK cells. * **CD25:** This is the alpha chain of the **IL-2 receptor**. While it can be expressed on activated NK cells, it is classically the hallmark of **Regulatory T cells (Tregs)** and activated T/B cells. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Markers:** Always look for **CD16 and CD56**. Notably, NK cells are **CD3 negative** (distinguishing them from NKT cells). * **Function:** NK cells provide the first line of defense against viral infections and tumor cells by recognizing the **absence of MHC Class I** (the "Missing Self" hypothesis). * **Cytokines:** NK cell activity is significantly enhanced by **IL-2 and IL-12**. * **Clinical Correlation:** Deficiencies in NK cell function or markers like CD16 are associated with recurrent viral infections (especially Herpesviruses).

Question 95: Which method is best for assessing hormone levels?

A. Flow cytometry
B. Electrophoresis
C. ELISA
D. RIA (Correct Answer)

Explanation: **Explanation:** The assessment of hormone levels requires a technique with extreme **sensitivity and specificity**, as hormones circulate in the blood in minute quantities (often in nanogram or picogram concentrations). **Why RIA is the Correct Answer:** **Radioimmunoassay (RIA)** is considered the gold standard for hormone estimation due to its unparalleled sensitivity. It is based on the principle of **competitive binding**, where a labeled antigen (radioactive) competes with an unlabeled antigen (patient sample) for a limited number of antibody binding sites. The high sensitivity of RIA makes it ideal for detecting substances present in trace amounts, such as thyroid hormones (T3, T4), insulin, and steroid hormones. **Analysis of Incorrect Options:** * **A. Flow Cytometry:** Primarily used for cell analysis (e.g., CD4/CD8 counts in HIV) and detecting cell surface markers. It is not used for quantifying soluble hormones. * **B. Electrophoresis:** Used to separate proteins based on size and charge (e.g., Serum Protein Electrophoresis for Multiple Myeloma). It lacks the sensitivity required for hormone quantification. * **C. ELISA:** While ELISA is commonly used in labs for hormones (like TSH or hCG) because it is safer (no radiation), **RIA remains technically superior in terms of sensitivity** for very low-concentration analytes. In competitive exams, RIA is the preferred answer for "best method" for hormones unless specified otherwise. **High-Yield Clinical Pearls for NEET-PG:** * **RIA Principle:** Competitive binding (Saturation analysis). * **ELISA Principle:** Antigen-antibody interaction with an enzyme-linked color change. * **Most Sensitive Method:** RIA > ELISA. * **Safety:** ELISA is preferred in modern clinical practice to avoid the hazards of radioactive waste associated with RIA (e.g., Iodine-125).

Question 96: Which toll-like receptors are involved in the action of bacterial endotoxins?

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

Explanation: **Explanation:** The correct answer is **Option C (TLR-4)**. **Why TLR-4 is correct:** Toll-like receptors (TLRs) are a class of Pattern Recognition Receptors (PRRs) that recognize conserved microbial structures known as Pathogen-Associated Molecular Patterns (PAMPs). **TLR-4** is specifically responsible for recognizing **Lipopolysaccharide (LPS)**, which is the primary component of the **endotoxin** found in the outer membrane of Gram-negative bacteria. When LPS binds to TLR-4 (with the help of MD2 and CD14), it triggers a signaling cascade (via NF-κB) that leads to the release of pro-inflammatory cytokines (TNF-α, IL-1, IL-6), potentially resulting in septic shock. **Why other options are incorrect:** * **TLR-1:** Usually forms a heterodimer with TLR-2 to recognize triacylated lipopeptides (found in bacteria and mycobacteria). * **TLR-2:** Primarily recognizes **Peptidoglycan**, lipoteichoic acid (Gram-positive bacteria), and fungal zymosan. * **TLR-3:** Located on endosomal membranes; it recognizes **double-stranded RNA (dsRNA)**, which is characteristic of certain viruses. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-5:** Recognizes **Flagellin** (bacterial flagella). * **TLR-7 & 8:** Recognize single-stranded RNA (ssRNA). * **TLR-9:** Recognizes unmethylated **CpG DNA** (bacterial/viral DNA). * **Location:** TLRs 1, 2, 4, 5, and 6 are found on the **cell surface**, while TLRs 3, 7, 8, and 9 are located in **endosomes** (intracellular). * **Genetic Link:** Mutations in the TLR-4 gene are associated with increased susceptibility to Gram-negative septicemia.

Question 97: Which of the following is a cytophilic antibody?

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

Explanation: **Explanation:** **Cytophilic antibodies** are immunoglobulins that bind to the surface of specific cells (such as mast cells, basophils, or macrophages) via their **Fc region** before encountering an antigen. **Why IgE is the Correct Answer:** IgE is the classic example of a cytophilic antibody. It has a very high affinity for **FcεRI receptors** located on the membranes of **mast cells and basophils**. Once bound, the IgE remains on the cell surface for weeks. When an allergen later cross-links these bound IgE molecules, it triggers immediate degranulation and the release of inflammatory mediators (histamine), leading to **Type I Hypersensitivity** reactions. **Analysis of Incorrect Options:** * **IgM:** It is the largest antibody (pentamer) and the first to appear in primary response. It is not cytophilic; it primarily activates the classical complement pathway. * **IgA:** Known as the "secretory antibody," it provides mucosal immunity. It exists as a dimer in secretions and does not typically bind to cell surfaces as a primary function. * **IgG:** While certain subclasses of IgG can bind to phagocytes (opsonization), it is generally not classified as a "cytophilic antibody" in the context of Type I hypersensitivity. It is the only antibody that crosses the placenta. **High-Yield Clinical Pearls for NEET-PG:** * **Prausnitz-Küstner (PK) Reaction:** Historically used to demonstrate the cytophilic nature of IgE by transferring serum from an allergic individual to a non-allergic one. * **Heat Lability:** IgE is heat-labile (inactivated at 56°C for 30 minutes), unlike IgG. * **Reaginic Antibody:** IgE is also known as the reaginic antibody. * **Parasitic Infections:** IgE levels are characteristically elevated in helminthic infections (Eosinophilia-Myalgia syndrome).

Question 98: Immune tolerance can be induced by?

A. Excess antigen (Correct Answer)
B. Excess antibody
C. Excess complement level
D. Neonatal thymectomy

Explanation: **Explanation:** **Immune tolerance** is a state of specific unresponsiveness to an antigen. The correct answer is **Excess antigen** because the concentration and persistence of an antigen are critical factors in determining whether an immune response or tolerance occurs. 1. **Why Excess Antigen is Correct:** High doses of an antigen can lead to **High-zone tolerance**, where both B-cells and T-cells become unresponsive. This occurs because an overwhelming amount of antigen can cause receptor blockade or trigger clonal exhaustion/anergy. Conversely, repeated very low doses can lead to **Low-zone tolerance** (primarily affecting T-cells). 2. **Why the other options are incorrect:** * **Excess antibody:** While antibodies can regulate immune responses via feedback inhibition (e.g., Rhogam), they do not "induce" immunological tolerance in the classical sense; rather, they mask epitopes or bind to inhibitory receptors (FcγRIIB). * **Excess complement level:** Complements are mediators of the innate immune system and enhancers of inflammation. High levels generally promote immune activation and clearance, not tolerance. * **Neonatal thymectomy:** The thymus is essential for the development of **Central Tolerance** (negative selection). Removing the thymus in a neonate prevents the maturation of T-cells, leading to severe immunodeficiency (similar to DiGeorge syndrome) rather than inducing a state of specific tolerance. **High-Yield NEET-PG Pearls:** * **Anergy:** Functional inactivation of lymphocytes due to lack of co-stimulatory signals (CD28-B7 interaction). * **Central Tolerance:** Occurs in primary lymphoid organs (Thymus/Bone marrow) via clonal deletion. * **Peripheral Tolerance:** Occurs in secondary lymphoid organs via anergy, suppression by T-regs, or clonal deletion. * **Tolerogens:** Antigens that induce tolerance rather than an immune response (usually soluble, high-dose, or administered intravenously/orally).

Question 99: Which statement about pattern recognition receptors is false?

A. Toll-like receptors are the best-known pattern recognition receptors.
B. Toll-like receptors are present in the plasma membrane and activate NF-κB and interferon regulatory factor.
C. NOD-like receptors (NLRs) are cytosolic receptors, and their mutations can lead to autoimmune disorders. (Correct Answer)
D. RIG-like receptors (RLRs) are located in the cytosol and stimulate the production of antiviral cytokines.

Explanation: **Explanation:** Pattern Recognition Receptors (PRRs) are germline-encoded receptors of the innate immune system that detect Pathogen-Associated Molecular Patterns (PAMPs) and Damage-Associated Molecular Patterns (DAMPs). **Why Option C is the correct (False) statement:** While NOD-like receptors (NLRs) are indeed cytosolic receptors, mutations in NLRs are primarily associated with **autoinflammatory syndromes** (e.g., Blau syndrome, Muckle-Wells syndrome) rather than classic autoimmune disorders. Autoinflammatory diseases involve dysfunction of the innate immune system (excessive IL-1 production via the inflammasome), whereas autoimmune diseases involve a breakdown of adaptive immunity (T and B cell tolerance). **Analysis of other options:** * **Option A:** Toll-like receptors (TLRs) are the most extensively studied and well-characterized PRRs. * **Option B:** TLRs are located on the plasma membrane (detecting bacteria/fungi) or endosomal membranes (detecting nucleic acids). Their signaling pathways culminate in the activation of **NF-κB** (pro-inflammatory) and **IRFs** (antiviral). * **Option D:** RIG-like receptors (RLRs) are cytosolic sensors that specifically detect viral RNA, triggering the production of **Type I Interferons (IFN-α/β)**. **High-Yield Facts for NEET-PG:** * **TLR-4** recognizes Lipopolysaccharide (LPS) of Gram-negative bacteria (requires MD2 and CD14). * **TLR-3** is the only TLR that does not use the MyD88 signaling pathway. * **Inflammasomes:** Multi-protein complexes (often involving NLRP3) that activate Caspase-1, leading to the cleavage and release of **IL-1β and IL-18**. * **Location Summary:** TLRs (Surface/Endosome), NLRs (Cytosol), RLRs (Cytosol), CLRs (Surface).

Question 100: The VDRL test is used for the diagnosis of syphilis. What type of immunological test is it?

A. Precipitation test
B. Agglutination test
C. Immunofluorescence test
D. Flocculation test (Correct Answer)

Explanation: **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is a non-specific, non-treponemal screening test for syphilis. It detects **reagin antibodies** (IgM and IgG) produced against cardiolipin-cholesterol-lecithin antigen. **Why D is correct:** The VDRL test is a specific type of **Precipitation test** known as **Flocculation**. In this reaction, the antigen is present in a fine particulate form. When it reacts with the antibody in the patient's serum, the resulting antigen-antibody complexes do not sediment but remain suspended as visible **flakes or "floccules."** Because these particles are microscopic, the VDRL test must be read under a light microscope (10x magnification). **Why other options are incorrect:** * **A. Precipitation test:** While flocculation is a subtype of precipitation, "Flocculation" is the more precise and clinically accurate term for VDRL. In standard precipitation, the product usually settles at the bottom. * **B. Agglutination test:** This involves the clumping of **insoluble/particulate antigens** (like RBCs or bacteria). VDRL uses soluble lipid antigens, making it a precipitation-based reaction. (Note: The RPR test is a modified flocculation test using charcoal particles for macroscopic viewing). * **C. Immunofluorescence test:** This uses fluorescent dyes (e.g., FTA-ABS). VDRL uses simple light microscopy. **High-Yield Clinical Pearls for NEET-PG:** * **Antigen used:** Cardiolipin (extracted from beef heart), cholesterol, and lecithin. * **Specimen:** VDRL can be used for both **Serum** and **CSF** (Neurosyphilis). RPR is used only for serum. * **Biological False Positives (BFP):** Conditions like SLE, Leprosy, Malaria, and pregnancy can cause false positives. * **Prozone Phenomenon:** Very high antibody titers can lead to a false-negative result; serum must be diluted to get a positive reaction.

Practice by Chapter

Cells and Organs of Immune System

Practice Questions

Innate Immunity

Practice Questions

Adaptive Immunity

Practice Questions

Antigens and Antibodies

Practice Questions

Major Histocompatibility Complex

Practice Questions

Complement System

Practice Questions

Cytokines and Chemokines

Practice Questions

Hypersensitivity Reactions

Practice Questions

Autoimmunity and Autoimmune Diseases

Practice Questions

Immunodeficiency Disorders

Practice Questions

Transplantation Immunology

Practice Questions

Tumor Immunology

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?