Immunology Practice Questions

Q: Immunoglobulin is not used in the prophylaxis of which of the following conditions?

Influenza. **Explanation:** The correct answer is **Influenza**. Immunoglobulins (passive immunization) are used to provide immediate, short-term protection against specific infections, particularly after exposure. However, for **Influenza**, prophylaxis is achieved through **active immunization (vaccines)** or **antiviral chemoprophylaxis** (e.g., Oseltamivir). Immunoglobulin therapy is not a standard clinical practice for Influenza because the virus undergoes frequent antigenic drift and shift, and the primary defense mechanism required is local mucosal immunity (IgA) and T-cell response, which systemic IgG administration does not effectively provide. **Analysis of Options:** * **Hepatitis A:** Normal Human Immunoglobulin (NHIG) is used for pre-exposure and post-exposure prophylaxis (within 2 weeks) in non-immune individuals traveling to endemic areas or following close contact. * **Varicella:** Varicella-Zoster Immunoglobulin (VZIG) is indicated for post-exposure prophylaxis in high-risk individuals (e.g., immunocompromised, pregnant women, or neonates) to prevent or modify the disease. * **Measles:** NHIG can prevent or modify measles if given within 6 days of exposure, especially in infants under 9 months or immunocompromised contacts. **NEET-PG High-Yield Pearls:** * **Passive-Active Immunization:** For Rabies, Hepatitis B, and Tetanus, both immunoglobulin and vaccine are given simultaneously at different sites. * **NHIG vs. Specific IG:** NHIG is used for Hepatitis A and Measles. Specific (Hyperimmune) IGs are used for Hepatitis B (HBIG), Rabies (RIG), Tetanus (TIG), and Varicella (VZIG). * **Contraindication:** Live vaccines (like MMR or Varicella) should generally be delayed for 3–11 months after receiving immunoglobulin therapy to prevent interference with the immune response.

Q: A deficiency of which of the following cells can predispose to candidiasis?

T Lymphocytes. **Explanation:** The correct answer is **T Lymphocytes**. The body’s defense against *Candida albicans* is a dual mechanism involving both innate and adaptive immunity. 1. **Why T Lymphocytes are correct:** Cell-Mediated Immunity (CMI), specifically **Th17 and Th1 cells**, is crucial for controlling mucocutaneous fungal infections. T lymphocytes produce cytokines (like IL-17 and IL-22) that recruit neutrophils and maintain mucosal integrity. A deficiency in T-cells (as seen in HIV/AIDS or Chronic Mucocutaneous Candidiasis) leads to persistent **mucocutaneous candidiasis** (oral thrush, esophagitis). 2. **Why other options are incorrect:** * **Eosinophils:** These are primarily involved in defense against helminthic (parasitic) infections and type I hypersensitivity reactions; they play a negligible role in fungal defense. * **Macrophages:** While they act as antigen-presenting cells and can ingest yeast, they are not the primary cell type whose *deficiency* characteristically predisposes to candidiasis compared to T-cells or neutrophils. * **Plasma cells:** These produce antibodies (Humoral Immunity). While antibodies can prevent systemic spread, they are not the primary defense against the localized mucosal colonization of *Candida*. **Clinical Pearls for NEET-PG:** * **Dual Defense Rule:** * **T-cell deficiency** predisposes to **Mucocutaneous Candidiasis** (e.g., HIV patients). * **Neutrophil deficiency** (Neutropenia) predisposes to **Disseminated/Systemic Candidiasis** (e.g., chemotherapy patients). * **Th17 cells** are the specific subset of T-cells most vital for fungal immunity at mucosal surfaces. * **Chronic Mucocutaneous Candidiasis (CMC)** is often associated with AIRE gene mutations or STAT1 signaling defects.

Q: What is the most important Human Leukocyte Antigen (HLA) for organ transplantation and tissue typing?

HLA-D. **Explanation:** The Major Histocompatibility Complex (MHC) in humans, known as the **Human Leukocyte Antigen (HLA)** system, is divided into Class I (A, B, C) and Class II (DR, DQ, DP). **Why HLA-D is the Correct Answer:** HLA-D (specifically the **HLA-DR** locus) is considered the most critical for organ transplantation and tissue typing. This is because Class II antigens are primarily responsible for initiating the **Mixed Lymphocyte Reaction (MLR)** and the proliferation of T-helper cells. In clinical transplantation, a mismatch at the HLA-D/DR locus is the strongest predictor of acute graft rejection and Graft-versus-Host Disease (GVHD). Therefore, matching at the D-locus is prioritized over Class I matching to ensure long-term graft survival. **Analysis of Incorrect Options:** * **HLA-A & HLA-B:** These are Class I MHC molecules. While they are important for tissue typing and are routinely matched (especially in kidney transplants), they are generally considered less immunogenic than HLA-DR in the context of initiating the primary immune response against a graft. * **HLA-C:** This is also a Class I molecule, but it has the lowest level of polymorphism and clinical significance among the major HLA loci in routine transplantation. **NEET-PG High-Yield Pearls:** * **MHC Class I (A, B, C):** Present endogenous antigens to **CD8+** T-cells. * **MHC Class II (DR, DQ, DP):** Present exogenous antigens to **CD4+** T-cells. * **Best Match:** Identical twins provide a perfect HLA match. Among siblings, there is a **25% (1 in 4)** chance of a complete HLA match. * **Ankylosing Spondylitis:** Strongly associated with **HLA-B27**. * **Narcolepsy:** Strongly associated with **HLA-DR2/DQB1**.

Q: Paul Bunnell antibodies are reactive in all except?

Ox red blood cells. The **Paul-Bunnell test** is a classic diagnostic tool used to detect **Infectious Mononucleosis (IM)** caused by the Epstein-Barr virus (EBV). The test identifies **heterophile antibodies**, which are IgM antibodies produced during IM that have the unique property of agglutinating red blood cells (RBCs) from different animal species. ### Why "Ox red blood cells" is the correct answer: Paul-Bunnell antibodies are defined by their ability to agglutinate **Sheep, Horse, and Dog RBCs**. However, they are **specifically absorbed by Ox (Beef) RBCs**. In the laboratory, if the patient's serum is pre-treated with Ox RBCs, the Paul-Bunnell antibodies are removed, and agglutination will no longer occur. Therefore, they are considered **reactive against** sheep/horse/dog cells but are **neutralized/absorbed by** ox cells. ### Analysis of Options: * **B, C, and D (Sheep, Dog, Horse RBCs):** These are the target antigens. Paul-Bunnell antibodies show positive agglutination reactions with these cells. Horse RBCs are currently the preferred substrate for the rapid "Monospot" test due to higher sensitivity. ### High-Yield Clinical Pearls for NEET-PG: 1. **Heterophile Antibodies:** These are "multispecific" antibodies that react with antigens across species boundaries but are **not** specific to EBV viral antigens. 2. **Differential Absorption (Davidsohn Differential Test):** * **IM Antibodies:** Absorbed by **Ox RBCs**; NOT absorbed by **Guinea pig kidney cells**. * **Forssman Antibodies:** Absorbed by **Guinea pig kidney cells**; NOT absorbed by Ox RBCs. * **Serum Sickness Antibodies:** Absorbed by **BOTH** Ox RBCs and Guinea pig kidney cells. 3. **Age Factor:** The Paul-Bunnell test is often **negative** in children under 5 years old with EBV infection.

Q: Which of the following cells expresses the CD3 receptor?

T cells. **Explanation:** **CD3 (Cluster of Differentiation 3)** is a definitive lineage marker for **T cells**. It is a protein complex composed of four distinct chains (gamma, delta, and two epsilon chains) that associate non-covalently with the **T-cell receptor (TCR)**. Its primary physiological role is signal transduction; while the TCR recognizes the antigen-MHC complex, the CD3 complex transmits the activation signal into the cytoplasmic compartment of the T cell. Because CD3 is required for the surface expression of the TCR, it is present on all mature T lymphocytes (both Helper T cells/CD4+ and Cytotoxic T cells/CD8+). **Analysis of Incorrect Options:** * **B cells:** These are characterized by markers such as **CD19, CD20, and CD21**. They express Surface Immunoglobulins (sIg) rather than TCR/CD3 complexes. * **Macrophages:** These are myeloid lineage cells. Their characteristic markers include **CD14** (receptor for LPS) and **CD16/CD64**. * **Eosinophils:** These are granulocytes identified by their bilobed nuclei and eosinophilic granules. They do not express lymphoid markers like CD3. **High-Yield Clinical Pearls for NEET-PG:** * **Pan-T cell marker:** CD3 is considered the most reliable "Pan-T cell marker" in immunohistochemistry to identify T-cell lymphomas. * **Muromonab-CD3 (OKT3):** A monoclonal antibody directed against CD3 used as an immunosuppressant to prevent acute organ transplant rejection. * **Double Negative/Positive:** During thymic T-cell maturation, cells start as CD4-/CD8- (Double Negative), then become CD4+/CD8+ (Double Positive) before maturing into single-positive T cells; however, **CD3 expression** increases as they mature.

Q: Which of the following is a function of Interleukin-1 (IL-1)?

T lymphocyte activation. **Explanation:** Interleukin-1 (IL-1) is a key pro-inflammatory cytokine primarily produced by activated macrophages and monocytes. It plays a central role in the "dual-signal" model of immune activation. **1. Why Option A is Correct:** IL-1 acts as a potent co-stimulator for **T lymphocyte activation**. When an Antigen-Presenting Cell (APC) presents an antigen to a T-cell via the MHC-TCR complex, it also releases IL-1. This cytokine induces T-cells to produce **Interleukin-2 (IL-2)** and express IL-2 receptors, leading to T-cell proliferation and the transition from the G0 to G1 phase of the cell cycle. **2. Why the Other Options are Incorrect:** * **Option B:** IL-1 actually **promotes wound healing** by stimulating the proliferation of fibroblasts and the synthesis of collagen. * **Option C:** While IL-1 is involved in the inflammatory cascade, it does not directly increase pain perception in the same way prostaglandins or bradykinin do; its primary systemic effects are fever and acute-phase reactant synthesis. * **Option D:** IL-1 causes **increased release of PMNs (neutrophils)** from the bone marrow, often leading to a "left shift" (neutrophilia with immature forms) during acute infection. **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Pyrogen:** IL-1 is the primary mediator of fever. It acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2) production, raising the thermoregulatory set-point. * **Acute Phase Response:** Along with IL-6 and TNF-α, IL-1 stimulates the liver to produce acute-phase proteins (e.g., CRP, Fibrinogen). * **Osteoclast Activation:** In the context of bone, IL-1 is also known as **Osteoclast Activating Factor (OAF)**, leading to bone resorption. * **Anakinra:** This is a recombinant IL-1 receptor antagonist used clinically in Rheumatoid Arthritis.

Q: T cells mature in:

Thymus. **Explanation:** The correct answer is **Thymus (Option C)**. **1. Why Thymus is correct:** T lymphocytes (T cells) originate from hematopoietic stem cells in the **bone marrow** but must migrate to the **thymus** to undergo maturation and differentiation. In the thymus, immature T cells (thymocytes) undergo two critical selection processes: * **Positive Selection:** Ensures T cells can recognize the body's MHC molecules. * **Negative Selection:** Eliminates self-reactive T cells to prevent autoimmunity. Once matured, they express either CD4 or CD8 markers and migrate to secondary lymphoid organs. **2. Why other options are incorrect:** * **Thyroid (A):** An endocrine gland responsible for metabolism via T3 and T4 hormones; it has no role in lymphocyte maturation. * **Tongue (B):** A muscular organ involved in gustation and deglutition. * **Trachea (D):** A cartilaginous tube serving as the primary airway for the respiratory system. **3. NEET-PG High-Yield Pearls:** * **Primary Lymphoid Organs:** Bone marrow (B-cell maturation) and Thymus (T-cell maturation). * **DiGeorge Syndrome:** A classic exam topic where thymic hypoplasia leads to T-cell deficiency and recurrent viral/fungal infections. * **Hassall’s Corpuscles:** These are characteristic histological features found in the medulla of the thymus. * **Involution:** The thymus is most active during childhood and undergoes fatty replacement (involution) after puberty, though T-cell production continues at a lower rate.

Q: Which of the following is an example of Type-II hypersensitivity?

Serum sickness. **Explanation:** The question asks for an example of **Type II hypersensitivity**, but there is a discrepancy in the provided key: **Serum sickness is actually a classic example of Type III hypersensitivity.** In Type III reactions, soluble antigen-antibody complexes circulate and deposit in tissues (like joints and kidneys), activating complement and causing systemic inflammation. **Why the options are classified as follows:** * **B. Hemolytic Anemia & D. Goodpasture Syndrome:** Both are classic **Type II (Cytotoxic) hypersensitivity** reactions. In Type II, antibodies (IgG/IgM) bind to antigens on specific cell surfaces or extracellular matrix, leading to cell destruction via complement activation or ADCC. * *Hemolytic Anemia:* Antibodies target RBC surfaces. * *Goodpasture Syndrome:* Antibodies target the glomerular basement membrane (anti-GBM). * **C. Serum Sickness (Marked Correct):** This is **Type III**. It occurs when foreign proteins (antigens) are injected, leading to widespread immune-complex deposition. * **A. Contact Dermatitis:** This is **Type IV (Delayed-type)** hypersensitivity, mediated by T-cells rather than antibodies. **Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (ACID):** * **A**naphylactic (Type I): IgE-mediated (Asthma, Urticaria). * **C**ytotoxic (Type II): Antibody against cell surface (Myasthenia Gravis, Rheumatic Fever). * **I**mmune-Complex (Type III): Soluble complexes (SLE, Post-streptococcal GN, Arthus reaction). * **D**elayed (Type IV): T-cell mediated (Mantoux test, Graft rejection). * **High-Yield Note:** If this question appeared in an exam with "Serum Sickness" as the key, it is likely a technical error in the question bank, as both B and D are technically correct examples of Type II.

Question 1

Immunoglobulin is not used in the prophylaxis of which of the following conditions?

Accepted Answer

Influenza

Answer

Hepatitis A

Answer

Varicella

Answer

Measles

Question 2

A deficiency of which of the following cells can predispose to candidiasis?

Accepted Answer

T Lymphocytes

Answer

Eosinophils

Answer

Macrophages

Answer

Plasma cells

Question 3

Which of the following events occurs first in the differentiation sequence of human B cells in the bone marrow?

Accepted Answer

Immunoglobulin heavy chain rearrangement

Answer

Cytoplasmic mu chains present in the B cell

Answer

Immunoglobulin light chain rearrangement

Answer

Surface IgD and IgM present on the B cell

Question 4

What is the most important Human Leukocyte Antigen (HLA) for organ transplantation and tissue typing?

Accepted Answer

HLA-D

Answer

HLA-A

Answer

HLA-B

Answer

HLA-C

Question 5

Paul Bunnell antibodies are reactive in all except?

Accepted Answer

Ox red blood cells

Answer

Sheep red blood cells

Answer

Dog red blood cells

Answer

Horse red blood cells

Question 6

Which of the following cells expresses the CD3 receptor?

Accepted Answer

T cells

Answer

B cells

Answer

Macrophages

Answer

Eosinophils

Question 7

Which of the following is a function of Interleukin-1 (IL-1)?

Accepted Answer

T lymphocyte activation

Answer

Delayed wound healing

Answer

Increased pain perception

Answer

Decreased PMN release from bone marrow

Question 8

T cells mature in:

Accepted Answer

Thymus

Answer

Thyroid

Answer

Tongue

Answer

Trachea

Question 9

Which of the following is an example of Type-II hypersensitivity?

Accepted Answer

Serum sickness

Answer

Contact dermatitis

Answer

Hemolytic anemia

Answer

Good pasture syndrome

Question 10

Which of the following is an example of a heterophile antigen?

Accepted Answer

Forssman antigen

Answer

Cryptococcus polysaccharide

Answer

Protein A of Staphylococcus

Answer

All of the above

Immunology — MCQs

Immunology — MCQs

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