Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 51: Complement is a series of important host proteins which provide protection from invasion by foreign microorganisms. Which one of the following statements best describes complement?

A. Complement inhibits phagocytosis
B. Microorganisms agglutinate in the presence of complement but do not lyse
C. Complement plays a minor role in the inflammatory response
D. Complement protects the host from pneumococcal and Haemophilus infection through complement components C1, C2, and C4 (Correct Answer)

Explanation: **Explanation:** The complement system is a vital component of innate immunity, consisting of plasma proteins that act in a cascade to eliminate pathogens. **Why Option D is Correct:** The **Classical Pathway** (initiated by C1, C2, and C4) is essential for the defense against encapsulated bacteria like *Streptococcus pneumoniae* and *Haemophilus influenzae*. These bacteria possess polysaccharide capsules that inhibit direct phagocytosis. The classical pathway, often triggered by antibodies (IgM or IgG) binding to the capsule, leads to the deposition of **C3b (opsonization)**. This allows phagocytes to recognize and ingest the bacteria. Patients with deficiencies in early classical components (C1, C2, C4) are clinically predisposed to recurrent infections with these specific pyogenic organisms. **Why Other Options are Incorrect:** * **Option A:** Incorrect. Complement **promotes** phagocytosis, primarily through C3b and C4b acting as opsonins. * **Option B:** Incorrect. While complement can assist in agglutination, its primary effector functions are **lysis** (via the Membrane Attack Complex, C5b-9), opsonization, and chemotaxis. * **Option C:** Incorrect. Complement plays a **major** role in inflammation. Small fragments like **C3a, C4a, and C5a (Anaphylatoxins)** trigger mast cell degranulation, while C5a is a potent chemoattractant for neutrophils. **NEET-PG High-Yield Pearls:** * **Most common complement deficiency:** C2 deficiency. * **C3 deficiency:** Most severe; leads to recurrent pyogenic infections and Type III hypersensitivity. * **C5-C9 (MAC) deficiency:** Specifically predisposes to disseminated *Neisseria* infections. * **CH50 Assay:** Used to screen the total hemolytic complement activity (integrity of the classical pathway). * **Opsonization:** C3b is the most potent opsonin.

Question 52: The term MHC restriction refers to which of the following statements?

A. Inheritance of MHC molecules
B. Need for antigen to be recognized in association with MHC molecules (Correct Answer)
C. Need for MHC genes to control complement activity
D. Need for MHC molecules in order to reject grafts

Explanation: ### Explanation **MHC Restriction** is a fundamental principle of immunology describing how T cells recognize antigens. Unlike B cells, which can bind to free-floating antigens, T cells are "restricted": they can only recognize an antigen if it is processed into peptides and presented on the surface of an APC (Antigen Presenting Cell) or a target cell in complex with a **Major Histocompatibility Complex (MHC)** molecule. #### Why Option B is Correct: T cell receptors (TCRs) do not recognize the antigen alone; they recognize a specific combination of the **antigenic peptide + the MHC molecule**. This ensures that T cells only interact with cells that are presenting foreign material, preventing them from attacking healthy, free-floating proteins. #### Why Other Options are Incorrect: * **Option A:** MHC molecules are inherited as a haplotype (codominantly), but this describes **MHC inheritance**, not restriction. * **Option C:** While some complement components (C2, C4, Factor B) are encoded in the MHC Class III region, this is a genetic location fact, not "restriction." * **Option D:** Graft rejection occurs due to **MHC incompatibility** (allorecognition), where the recipient's T cells attack foreign MHC molecules. While MHC is involved, the term "restriction" specifically refers to the antigen-recognition process. --- ### High-Yield Clinical Pearls for NEET-PG: * **The Rule of 8:** * **MHC Class I** (found on all nucleated cells) restricts **CD8+** T cells (1 × 8 = 8). * **MHC Class II** (found only on APCs) restricts **CD4+** T cells (2 × 4 = 8). * **Endogenous vs. Exogenous:** MHC I typically presents endogenous antigens (e.g., viral proteins synthesized inside the cell), while MHC II presents exogenous antigens (e.g., phagocytosed bacteria). * **Zinkernagel and Doherty:** They won the Nobel Prize for discovering the phenomenon of MHC restriction.

Question 53: Which of the following organisms does NOT produce a superantigen?

A. Mycoplasma arthridis
B. Enterotoxin A of food poisoning
C. Malassezia furfur
D. Enterococcus fecalis (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the mechanism of **Superantigens (SAgs)**. Unlike conventional antigens, superantigens do not undergo intracellular processing. Instead, they bind directly to the **MHC class II** molecules on Antigen-Presenting Cells (APCs) and the **Vβ chain of T-cell receptors (TCR)**. This bypasses the specificity of the immune response, leading to the massive activation of up to 20% of the body’s T-cells and a subsequent "cytokine storm" (IFN-γ, IL-1, IL-6, and TNF-α). * **Why Enterococcus faecalis is correct:** While *E. faecalis* is a significant human pathogen causing UTIs, endocarditis, and biliary tract infections, it is **not** known to produce superantigens. Its virulence factors primarily include surface proteins (aggregation substance), gelatinase, and cytolysin. * **Why the other options are incorrect:** * **Mycoplasma arthritidis:** Produces the *M. arthritidis* mitogen (MAM), a well-documented superantigen. * **Enterotoxin A (Staphylococcal):** Produced by *Staphylococcus aureus*, this is a classic superantigen responsible for staphylococcal food poisoning and toxic shock-like symptoms. * **Malassezia furfur:** This fungus produces superantigens that contribute to the inflammatory response in conditions like seborrheic dermatitis and atopic dermatitis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Classic Examples of SAgs:** TSST-1 (*S. aureus*), Pyrogenic exotoxin A and C (*S. pyogenes*), and Exfoliative toxin (Scalded Skin Syndrome). 2. **Viral Superantigens:** Epstein-Barr Virus (EBV), Cytomegalovirus (CMV), and Rabies virus nucleocapsid. 3. **Key Feature:** SAgs bind to the **variable region of the beta chain (Vβ)** of the T-cell receptor. 4. **Result:** Massive release of **TNF-α and IL-1**, leading to shock and multi-organ failure.

Question 54: Which of the following HLA types is associated with rheumatoid arthritis?

A. HLA B27
B. HLA DR4 (Correct Answer)
C. HLA B8
D. HLA DP

Explanation: **Explanation:** The association between Human Leukocyte Antigens (HLA) and specific diseases is a high-yield topic in NEET-PG. HLA molecules are responsible for presenting antigens to T-cells; certain alleles are more efficient at presenting self-antigens, leading to autoimmunity. **1. Why HLA-DR4 is correct:** Rheumatoid Arthritis (RA) is strongly associated with **HLA-DR4** (specifically the DRB1*0401 and *0404 alleles). These alleles contain a specific sequence of amino acids known as the **"Shared Epitope"** located in the peptide-binding groove of the HLA molecule. This structural feature allows for the presentation of citrullinated peptides to T-cells, triggering the inflammatory cascade characteristic of RA. **2. Analysis of Incorrect Options:** * **HLA-B27:** This is the classic association for **Seronegative Spondyloarthropathies**, including Ankylosing Spondylitis (strongest association), Reiter’s syndrome (Reactive Arthritis), Psoriatic arthritis, and Enteropathic arthritis. * **HLA-B8:** This is associated with **Myasthenia Gravis**, Graves' disease, and Celiac disease (along with DR3). * **HLA-DP:** While HLA-DP alleles are linked to some conditions like Berylliosis, they are not the primary diagnostic association for Rheumatoid Arthritis. **Clinical Pearls for NEET-PG:** * **HLA-DR3/DR4:** Both are associated with Type 1 Diabetes Mellitus. * **HLA-DR2:** Associated with Multiple Sclerosis, SLE, and Goodpasture syndrome. * **HLA-DQ2/DQ8:** Highly specific for Celiac Disease. * **Rule of Thumb:** Most HLA-DR associations involve autoimmune/rheumatological conditions, while HLA-B associations often involve spondyloarthropathies or skin conditions.

Question 55: Antisera is obtained from which animal?

A. Guinea pig
B. Horse (Correct Answer)
C. Rat
D. Rabbit

Explanation: **Explanation:** **Why Horse is the correct answer:** In medical immunology, **Antisera** (serum containing specific antibodies) is primarily produced for therapeutic use against toxins, venoms, and pathogens. The **Horse** is the preferred animal for large-scale production of heterologous antisera (e.g., Anti-Tetanus Serum, Anti-Diphtheritic Serum, and Anti-Snake Venom). The primary reason is the horse's **large body size**, which allows for the collection of significant volumes of blood (plasmapheresis) without harming the animal. Furthermore, horses are easy to handle, have a robust immune response, and produce high titers of antibodies. **Analysis of Incorrect Options:** * **A. Guinea pig:** While used in diagnostic labs (e.g., for the *C. diphtheriae* virulence test or as a source of complement), their small size makes them unsuitable for bulk antisera production. * **C. Rat:** Rats are primarily used in basic research and experimental immunology rather than for the commercial production of therapeutic antisera. * **D. Rabbit:** Rabbits are frequently used in laboratories to produce **polyclonal antibodies** for research and diagnostic assays (like ELISA or Western Blot), but they cannot provide the volume required for human clinical therapy. **Clinical Pearls for NEET-PG:** * **Serum Sickness:** Since horse serum is a foreign protein (heterologous), its administration can lead to Type III Hypersensitivity (Serum Sickness). * **Hyperimmunization:** The process of repeatedly injecting an animal with an antigen to produce high-titer antisera is called hyperimmunization. * **Refining:** Modern antisera are often "despeciated" (digested with pepsin) to remove the Fc fragment, reducing the risk of allergic reactions while retaining the antigen-binding F(ab')2 fragments.

Question 56: Bence Jones proteins are best described as:

A. Chains
B. G chains
C. Kappa & Lambda chains (Correct Answer)
D. Fibrin split products

Explanation: **Explanation:** **Bence Jones proteins (BJPs)** are monoclonal globulins consisting of **free immunoglobulin light chains** (either **Kappa or Lambda**). In healthy individuals, light chains are produced in slight excess of heavy chains, but they are usually reabsorbed by the renal tubules. In plasma cell dyscrasias, particularly **Multiple Myeloma**, there is a massive overproduction of these light chains that exceeds the renal threshold, leading to their excretion in the urine. **Analysis of Options:** * **Option C (Correct):** BJPs are specifically the free light chains (Kappa or Lambda) of immunoglobulins. They are unique because of their **thermal property**: they precipitate when heated to 40–60°C and redissolve upon boiling (100°C). * **Option A & B (Incorrect):** These refer to heavy chains (Alpha and Gamma). While heavy chains are part of the complete immunoglobulin structure, they do not constitute Bence Jones proteins. * **Option D (Incorrect):** Fibrin split products (FSPs) are degradation products of fibrinogen/fibrin, typically elevated in conditions like Disseminated Intravascular Coagulation (DIC), and have no relation to immunoglobulin production. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Test:** The standard urine dipstick (which detects albumin) often fails to detect BJPs. **Sulphosalicylic acid (SSA) test** or **Urine Protein Electrophoresis (UPEP)** is required for detection. * **Renal Impact:** BJPs are nephrotoxic and can lead to "Myeloma Kidney" (cast nephropathy). * **Ratio:** In Multiple Myeloma, the normal Kappa:Lambda ratio (approx. 2:1) is significantly altered. * **Amyloidosis:** BJPs are the precursors for **AL (Amyloid Light-chain) amyloidosis**.

Question 57: Which of the following features is not shared between T cells and B cells?

A. Positive selection during development (Correct Answer)
B. Class I MHC expression
C. Antigen-specific receptors
D. All of the above

Explanation: The correct answer is **A. Positive selection during development.** ### Explanation **1. Why Option A is correct:** While both T and B cells undergo **negative selection** (deletion of self-reactive cells to ensure self-tolerance), **positive selection** is a process unique to **T cell development** in the thymus. During positive selection, T cells must demonstrate the ability to bind to self-MHC molecules with moderate affinity. If they fail to recognize self-MHC, they undergo apoptosis. B cells do not require MHC recognition for their function; therefore, they do not undergo positive selection. **2. Why other options are incorrect:** * **Option B (Class I MHC expression):** All nucleated cells in the human body express MHC Class I molecules. Since both B cells and T cells are nucleated lymphocytes, they both express MHC Class I. (Note: B cells also express MHC Class II as they are professional Antigen Presenting Cells). * **Option C (Antigen-specific receptors):** Both cell types possess highly specific receptors generated by V(D)J recombination. B cells have **BCRs** (surface immunoglobulins), and T cells have **TCRs**. Both are designed to recognize specific epitopes. ### High-Yield Clinical Pearls for NEET-PG * **Site of Maturation:** B cells mature in the **Bone marrow**; T cells mature in the **Thymus**. * **Selection Process:** * **T cells:** Undergo both Positive selection (Cortex of thymus) and Negative selection (Medulla of thymus). * **B cells:** Undergo only Negative selection (Bone marrow). * **MHC Restriction:** T cells are "MHC restricted" (CD4 to MHC II, CD8 to MHC I), whereas B cells can recognize free, native antigens directly without MHC presentation. * **Clonal Deletion:** This is the primary mechanism for central tolerance in both B and T cells, occurring during negative selection.

Question 58: Red infarcts occur in which of the following organs?

A. Kidney
B. Lung (Correct Answer)
C. Spleen
D. Heart

Explanation: ### Explanation Infarction occurs due to tissue necrosis resulting from ischemia. Infarcts are classified into two types based on their color and the nature of the blood supply: **Red (Hemorrhagic)** and **White (Anemic)**. **Why the Lung is Correct:** Red infarcts occur in tissues with a **dual blood supply** or loose stroma that allows blood to collect in the necrotic area. The lung receives blood from both the **pulmonary arteries** and **bronchial arteries**. When a pulmonary artery branch is obstructed, the bronchial circulation continues to pump blood into the necrotic zone, but the damaged tissue cannot contain it, leading to hemorrhage. Other sites for red infarcts include the small intestine (dual supply/collaterals), brain (liquefactive necrosis), and tissues following venous torsion (e.g., testis or ovary). **Why Other Options are Incorrect:** * **A, C, and D (Kidney, Spleen, Heart):** These are solid organs with **end-arterial circulation** (single blood supply). When the primary artery is blocked, there is no secondary source of blood to fill the area. This results in a **White (Anemic) Infarct**, which appears pale and wedge-shaped. **High-Yield NEET-PG Pearls:** * **White Infarcts:** Occur in solid organs with end-arteries (Heart, Spleen, Kidney). * **Red Infarcts:** Occur in organs with dual blood supply (Lung, Liver, GI tract), loose tissues, or following venous occlusion. * **Morphology:** Most infarcts are wedge-shaped, with the apex pointing toward the site of vascular occlusion. * **Reperfusion Injury:** Red infarcts can also occur when blood flow is restored to a previously ischemic/necrotic area.

Question 59: Which immunoglobulin constitutes approximately 75% of total immunoglobulins in humans?

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

Explanation: **Explanation:** **IgG** is the most abundant class of immunoglobulins in human serum, accounting for approximately **75–80%** of the total pool. It is the primary antibody of the secondary immune response and is uniquely characterized by its ability to cross the placenta, providing passive immunity to the fetus. **Analysis of Options:** * **IgM (Option B):** Constitutes about 5–10% of total serum antibodies. It is the largest immunoglobulin (pentamer) and the first to appear in response to an initial antigen exposure (primary response). * **IgE (Option C):** Present in trace amounts (<0.01%). It is primarily involved in Type I hypersensitivity (allergic) reactions and provides defense against helminthic parasitic infections. * **IgA (Option D):** Constitutes about 10–15% of serum antibodies. It is the predominant immunoglobulin in external secretions (colostrum, saliva, tears, and respiratory/GI mucus), where it exists as a dimer. **High-Yield NEET-PG Pearls:** * **Mnemonic for Concentration:** **GAMED** (IgG > IgA > IgM > IgE > IgD). * **Half-life:** IgG has the longest half-life (approx. 23 days), making it ideal for long-term immunity. * **Subclasses:** IgG has four subclasses (IgG1-IgG4); IgG1 is the most abundant. * **Complement Activation:** IgM is the most efficient at activating the classical complement pathway, followed by IgG. * **Warm vs. Cold:** IgG is associated with "Warm" autoimmune hemolytic anemia, while IgM is associated with "Cold" agglutinin disease.

Question 60: Which of the following is NOT an antigen-presenting cell?

A. Astrocytes (Correct Answer)
B. Endothelial cells
C. Epithelial cells
D. Langerhans cells

Explanation: **Explanation:** Antigen-presenting cells (APCs) are specialized cells that process and present antigens via Major Histocompatibility Complex (MHC) molecules to T-cells. They are broadly categorized into **Professional APCs** (Dendritic cells, Macrophages, B-cells) and **Non-professional APCs** (various tissue cells). **Why Astrocytes are the correct answer:** While the central nervous system (CNS) was historically considered "immunologically privileged," it does contain APCs. However, **Microglia** are the primary resident professional APCs of the brain. **Astrocytes** are supportive glial cells; while they can express MHC-II under extreme inflammatory conditions in vitro, they are generally **not** considered functional APCs in a physiological or standard clinical context. In the context of NEET-PG, they are the least likely to function as APCs compared to the other options. **Analysis of other options:** * **Langerhans cells:** These are professional APCs (dendritic cells) found in the stratum spinosum of the epidermis. They are the most potent stimulators of naive T-cells. * **Endothelial cells:** These are well-recognized non-professional APCs. They express MHC-I and can be induced to express MHC-II, allowing them to present antigens to circulating T-lymphocytes. * **Epithelial cells:** Certain epithelial cells (e.g., thymic epithelial cells or intestinal epithelial cells) act as non-professional APCs to maintain local immune homeostasis. **High-Yield Clinical Pearls for NEET-PG:** * **Professional APCs:** Dendritic cells (most potent), Macrophages, and B-cells. They express **MHC-II** constitutively. * **MHC Restriction:** CD4+ T-cells recognize antigens with MHC-II; CD8+ T-cells recognize antigens with MHC-I. * **Langerhans Cells:** Contain characteristic **Birbeck granules** (tennis-racket shaped) on electron microscopy and are positive for **S-100 and CD1a**.

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Immunology — MCQs

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