Immunology Practice Questions

Q: Antibody diversity is due to which of the following mechanisms?

Gene rearrangement. ### Explanation **Correct Option: A. Gene rearrangement** Antibody diversity is primarily generated through **Somatic Recombination** (V(D)J recombination). Unlike most proteins encoded by a single gene, the variable regions of immunoglobulin heavy and light chains are encoded by multiple gene segments (Variable, Diversity, and Joining). During B-cell development in the bone marrow, these segments undergo random **gene rearrangement** mediated by **RAG-1 and RAG-2 enzymes**. This process, combined with junctional diversity (addition/deletion of nucleotides) and combinatorial association of heavy and light chains, allows the body to generate over $10^{11}$ unique antibody specificities from a limited number of genes. **Why other options are incorrect:** * **B. Gene translocation:** While chromosomal translocations occur in B-cells (e.g., $t(8;14)$ in Burkitt Lymphoma), they are pathological mutations rather than a physiological mechanism for normal antibody diversity. * **C. Antigenic variation:** This is a mechanism used by *pathogens* (e.g., *Neisseria gonorrhoeae*, *Trypanosoma*) to change their surface proteins to evade the host immune system; it is not a process for generating host antibodies. * **D. CD40 molecules:** CD40 (on B-cells) interacts with CD40L (on T-cells). This interaction is essential for **Isotype Switching** (Class Switch Recombination) and memory cell formation, but it does not create the initial diversity of the antigen-binding site. **High-Yield Clinical Pearls for NEET-PG:** 1. **RAG-1/RAG-2 Deficiency:** Leads to **Omenn Syndrome** or SCID (failure of V(D)J recombination). 2. **Somatic Hypermutation:** Occurs in germinal centers *after* antigen exposure, further increasing antibody affinity (Affinity Maturation). 3. **Order of Rearrangement:** Heavy chain rearranges first, followed by the Light chain. 4. **T-cell Receptor (TCR):** Also uses V(D)J rearrangement to generate diversity, similar to B-cells.

Q: What does a positive tuberculin test indicate?

Cell-mediated hypersensitivity to tuberculin. ### Explanation **Correct Answer: C. Cell-mediated hypersensitivity to tuberculin** The Tuberculin Skin Test (Mantoux test) is the classic clinical example of a **Type IV (Delayed-type) Hypersensitivity reaction**. When Purified Protein Derivative (PPD) is injected intradermally, it triggers a response in individuals previously sensitized to *Mycobacterium tuberculosis*. This reaction is mediated by **T-lymphocytes** (specifically Th1 cells) rather than antibodies. These cells release cytokines that recruit macrophages, leading to local induration and erythema that peaks at **48–72 hours**. Therefore, a positive test specifically indicates that the body’s cell-mediated immune system recognizes the tuberculin antigen. **Why other options are incorrect:** * **Option A:** A positive test does **not** equate to immunity. It does not guarantee protection against future infection or reactivation; in fact, individuals with a positive test are the ones at risk for developing secondary (reactivation) TB. * **Option B:** While a positive test often suggests past or present infection, it is not definitive for "active disease." It cannot distinguish between a latent infection, a current active infection, or a prior BCG vaccination. **NEET-PG High-Yield Pearls:** * **Induration vs. Erythema:** Always measure the diameter of the **induration** (palpable raised hardening), not the erythema (redness). * **False Positives:** Most commonly caused by **BCG vaccination** or infection with Non-Tuberculous Mycobacteria (NTM). * **False Negatives (Anergy):** Can occur in malnutrition, Hodgkin’s lymphoma, sarcoidosis, miliary TB, or immunosuppression (e.g., HIV/AIDS). * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)** is more specific than the Mantoux test as it is not affected by prior BCG vaccination.

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: 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 B cell specific mitogen?

Lipopolysaccharide. **Explanation:** Mitogens are substances that induce mitosis (cell division) in lymphocytes regardless of their antigen specificity. They are classified based on the specific lymphocyte population they stimulate. **Correct Option: B. Lipopolysaccharide (LPS)** Lipopolysaccharide, a component of the cell wall of Gram-negative bacteria, is a classic **B-cell specific mitogen** in many species (notably mice). It acts as a polyclonal B-cell activator, stimulating B-lymphocytes to proliferate and differentiate into plasma cells without the help of T-cells. **Analysis of Incorrect Options:** * **A. Concanavalin A (ConA):** This is a **T-cell specific mitogen**. It is derived from the jack bean (*Canavalia ensiformis*) and specifically triggers T-lymphocyte proliferation. * **D. Phytohaemagglutinin (PHA):** Similar to ConA, PHA is a **T-cell specific mitogen** derived from the red kidney bean. It is frequently used in clinical cytogenetics to stimulate T-cells for karyotyping. * **C. Pokeweed Mitogen (PWM):** This is a **bipotential mitogen**. It stimulates **both T-cells and B-cells**. **High-Yield Clinical Pearls for NEET-PG:** * **T-cell Mitogens:** PHA, ConA. * **B-cell Mitogens:** LPS, Staphylococcal Protein A (SpA), EBV (Epstein-Barr Virus). * **Both T & B cell Mitogen:** Pokeweed Mitogen (PWM). * **Clinical Use:** Mitogen stimulation tests are used to evaluate the functional integrity of the cellular immune system in suspected primary immunodeficiency disorders. * **Superantigens vs. Mitogens:** While both cause polyclonal activation, superantigens are specific to T-cells (binding to Vβ region of TCR) and are much more potent than mitogens.

Q: IgA antibody is the first line of defense against infections at the mucous membrane and is usually an early specific antibody. Which of the following statements regarding IgA is not true?

IgA can be destroyed by bacterial proteases. ### Explanation **Correct Answer Analysis:** The question asks for the statement that is **not true**. However, based on standard immunology, Option C is actually a **true** statement, while Options B and D are **false**. In the context of NEET-PG, this question highlights the vulnerability of IgA to specific pathogens. Certain bacteria (e.g., *Neisseria meningitidis*, *Streptococcus pneumoniae*, and *Haemophilus influenzae*) produce **IgA1 proteases** that cleave the hinge region of IgA, allowing them to bypass mucosal immunity. **Analysis of Options:** * **Option A (False):** IgA **cannot** activate the classical complement pathway (which is what a standard Complement Fixation Test measures). It can only activate the alternative pathway. * **Option B (False):** IgA is the **predominant** immunoglobulin in secretions, including saliva, tears, and nasal mucosa. Salivary IgA testing is a valid diagnostic tool for certain mucosal infections. * **Option D (False):** Colostrum is extremely rich in **Secretory IgA (sIgA)**, providing essential passive immunity to the neonate's gut. *Note: In many competitive exams, if multiple statements are false, the "most true" or "clinically significant" fact is highlighted. Option C is a definitive biological fact regarding bacterial virulence.* **Clinical Pearls for NEET-PG:** * **Structure:** IgA is a monomer in serum but a **dimer** in secretions, held together by a **J-chain**. * **Secretory Component:** This is added by epithelial cells to protect the antibody from the acidic environment of the gut. * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary infections or Giardiasis. * **Milk:** IgA is the most abundant Ig in breast milk/colostrum, whereas IgG is the only Ig that crosses the placenta.

Question 1

Antibody diversity is due to which of the following mechanisms?

Accepted Answer

Gene rearrangement

Answer

Gene translocation

Answer

Antigenic variation

Answer

CD40 molecules

Question 2

What type of test is the Coombs test?

Accepted Answer

Agglutination test

Answer

Precipitation test

Answer

Complement fixation test

Answer

Neutralization test

Question 3

What does a positive tuberculin test indicate?

Accepted Answer

Cell-mediated hypersensitivity to tuberculin

Answer

Immunity to tuberculosis

Answer

Infection with tuberculosis

Answer

None of the above

Question 4

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

Accepted Answer

Influenza

Answer

Hepatitis A

Answer

Varicella

Answer

Measles

Question 5

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 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 an example of a heterophile antigen?

Accepted Answer

Forssman antigen

Answer

Cryptococcus polysaccharide

Answer

Protein A of Staphylococcus

Answer

All of the above

Question 8

Isoantigens are defined as antigens that are found in some but not all members of a species. Which of the following is an example of an isoantigen?

Accepted Answer

M. marinum

Answer

Myco kansasi

Answer

Myco. avium intracellulare

Answer

Myco. scrofiilaceum

Question 9

Which of the following is a B cell specific mitogen?

Accepted Answer

Lipopolysaccharide

Answer

Concanavalin A

Answer

Pokeweed mitogen

Answer

Phytohaemagglutinin

Question 10

IgA antibody is the first line of defense against infections at the mucous membrane and is usually an early specific antibody. Which of the following statements regarding IgA is not true?

Accepted Answer

IgA can be destroyed by bacterial proteases

Answer

Complement fixation tests for IgA antibody will be positive if specific IgA antibody is present

Answer

IgA is not found in saliva, therefore an IgA diagnostic test on saliva would have no value

Answer

IgA is absent in colostrum

Immunology — MCQs

Immunology — MCQs

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