Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 101: Complement Fixation test is:

A. WIDAL
B. Coombs test
C. Wassermann reaction (Correct Answer)
D. VDRL

Explanation: **Explanation:** The **Wassermann reaction** is a classic example of a **Complement Fixation Test (CFT)**. It was historically used as a non-specific screening test for Syphilis. In this test, if specific antibodies (reagin) are present in the patient's serum, they react with the added antigen (cardiolipin) and "fix" (consume) the available complement. When indicator cells (sensitized sheep RBCs) are added later, no lysis occurs because the complement has already been used up, indicating a **positive** result. **Analysis of Incorrect Options:** * **WIDAL Test:** This is a **direct agglutination** test used for the diagnosis of Enteric (Typhoid) fever, detecting antibodies against O and H antigens of *Salmonella typhi*. * **Coombs Test (Antiglobulin Test):** This is a specialized **agglutination** test used to detect incomplete antibodies against RBCs (e.g., in Rh incompatibility or autoimmune hemolytic anemia). * **VDRL (Venereal Disease Research Laboratory):** While also used for Syphilis, VDRL is a **slide flocculation** test (a type of precipitation), not a CFT. It detects reaginic antibodies using a cardiolipin-cholesterol-lecithin antigen. **High-Yield Pearls for NEET-PG:** * **CFT Principle:** Lysis of indicator cells = Negative result; No lysis = Positive result. * **Other CFT Examples:** Viral infections (e.g., Influenza), Mycoplasma pneumonia, and certain fungal infections. * **VDRL vs. Wassermann:** VDRL is the modern screening standard because it is faster and easier to perform than the complex Wassermann CFT. * **Prozone Phenomenon:** Can cause false negatives in agglutination/precipitation tests (like Widal or VDRL) due to antibody excess.

Question 102: Which RNA polymerase transcribes microRNA?

A. RNA polymerase I
B. RNA polymerase II (Correct Answer)
C. RNA polymerase III
D. DNA polymerase

Explanation: **Explanation:** The synthesis of microRNA (miRNA) is a multi-step process that begins in the nucleus. The majority of miRNAs are transcribed by **RNA polymerase II**. These transcripts, known as primary miRNAs (pri-miRNAs), are characterized by a 5' cap and a 3' poly-A tail, similar to messenger RNA (mRNA). RNA polymerase II is the primary enzyme responsible for transcribing all protein-coding genes and several non-coding RNAs, including miRNA and most snRNA. **Analysis of Options:** * **RNA Polymerase I:** This enzyme is localized in the nucleolus and is exclusively responsible for transcribing the **pre-rRNA (45S)**, which is processed into 18S, 5.8S, and 28S ribosomal RNAs. * **RNA Polymerase III:** This enzyme transcribes small, stable RNAs, most notably **tRNA** and the **5S rRNA**. While a very small subset of miRNAs can be transcribed by Pol III, Pol II is the standard answer for medical examinations. * **DNA Polymerase:** This enzyme is involved in **DNA replication** and repair, not the transcription of RNA from a DNA template. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for RNA Pol I, II, III:** **R-M-T** (1-2-3) * Pol **I**: **R**ibosomal RNA * Pol **II**: **M**essenger RNA (and miRNA) * Pol **III**: **T**ransfer RNA * **Amanita phalloides (Death Cap Mushroom):** Contains **$\alpha$-amanitin**, which potently inhibits **RNA Polymerase II**, leading to severe hepatotoxicity. * **miRNA Function:** They regulate gene expression post-transcriptionally by binding to the 3' UTR of target mRNA, leading to translational repression or mRNA degradation.

Question 103: Which of the following is NOT an immune complex hypersensitivity reaction?

A. Goodpasture syndrome (Correct Answer)
B. Systemic Lupus Erythematosus (SLE)
C. Autoimmune Hemolytic Anemia (AIHA) reaction
D. Post-streptococcal glomerulonephritis

Explanation: **Explanation:** The question asks to identify the condition that is **NOT** a Type III (Immune Complex) hypersensitivity reaction. **1. Why Goodpasture Syndrome is the Correct Answer:** Goodpasture syndrome is a classic example of **Type II Hypersensitivity** (Antibody-mediated). In this condition, specific autoantibodies (anti-GBM antibodies) are directed against the alpha-3 chain of Type IV collagen in the glomerular and alveolar basement membranes. These antibodies bind directly to the fixed tissue antigens, showing a characteristic **"linear"** pattern on immunofluorescence. **2. Analysis of Incorrect Options:** * **Systemic Lupus Erythematosus (SLE):** This is a prototypical **Type III Hypersensitivity** disease. It involves the formation of circulating antigen-antibody complexes (e.g., DNA-anti-DNA) that deposit in various tissues (kidneys, joints, vessels), leading to complement activation and inflammation. * **Post-streptococcal Glomerulonephritis (PSGN):** This is a **Type III** reaction. It occurs when streptococcal antigen-antibody complexes deposit in the glomerular basement membrane, typically presenting with a **"lumpy-bumpy"** or granular immunofluorescence pattern. * **Autoimmune Hemolytic Anemia (AIHA):** While often classified as Type II, in the context of this specific question's standard NEET-PG framing, **Option A** remains the most definitive answer because Goodpasture is the "gold standard" for Type II (fixed antigen), whereas SLE and PSGN are strictly Type III. *(Note: If AIHA is listed, it is also Type II, but Goodpasture is the more frequent examiner favorite for this distinction).* **3. NEET-PG High-Yield Pearls:** * **Type II (Cytotoxic):** Antibody binds to **fixed** antigen on cell surfaces. (Mnemonic: **A**ntibody-mediated). Examples: Myasthenia Gravis, Graves' disease, Rheumatic fever. * **Type III (Immune Complex):** **Circulating** complexes deposit in tissues. (Mnemonic: **I**mmune-complex). Examples: Serum sickness, Arthus reaction, Farmer’s lung. * **Immunofluorescence Tip:** Linear pattern = Type II (Goodpasture); Granular pattern = Type III (PSGN/SLE).

Question 104: Which of the following is NOT a live vaccine?

A. BCG
B. Salk vaccine (Correct Answer)
C. Measles
D. Mumps

Explanation: The correct answer is **B. Salk vaccine**. ### **Explanation** The fundamental concept here is the classification of vaccines into **Live Attenuated** and **Killed (Inactivated)** types. * **Salk vaccine (IPV - Inactivated Poliovirus Vaccine):** This is a killed vaccine, produced by inactivating the virus using formaldehyde. Because the pathogen is dead, it cannot replicate in the host but still triggers an immune response. * **Sabin vaccine (OPV - Oral Polio Vaccine):** In contrast, Sabin is a live attenuated vaccine. A common NEET-PG mnemonic to remember this is: *"Salk = K for Killed; Sabin = Live."* ### **Analysis of Incorrect Options** * **A. BCG (Bacillus Calmette-Guérin):** A live attenuated bacterial vaccine derived from *Mycobacterium bovis*. It is the only live bacterial vaccine routinely used in the national schedule. * **C & D. Measles and Mumps:** Both are live attenuated viral vaccines. They are typically administered as part of the MMR (Measles, Mumps, Rubella) or MR combination. ### **High-Yield Clinical Pearls for NEET-PG** * **Live Vaccines Mnemonic:** *"**B**oy **R**omeo **G**ive **M**y **L**ove **S**picy **V**ictory **T**onight"* (BCG, Rotavirus, OPV, MMR, L-Ty21a, Smallpox/Sabin, Varicella, Yellow Fever). * **Contraindications:** Live vaccines are generally contraindicated in **pregnancy** and **immunocompromised** individuals (except HIV patients with CD4 counts >200). * **Killed Vaccines:** Examples include Salk (IPV), Hepatitis A, Rabies, and the Whole-cell Pertussis component. * **Storage:** Most live vaccines are heat-sensitive and must be stored in the "cold chain" (usually +2°C to +8°C).

Question 105: Which immunoglobulin is found predominantly in secretions?

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

Explanation: **Explanation:** **IgA (Immunoglobulin A)** is the correct answer because it is the primary antibody class found in mucosal secretions (saliva, tears, colostrum, breast milk, and gastrointestinal/respiratory secretions). It exists in two forms: a **monomer** in the serum and a **dimer** in secretions. The dimeric form is held together by a **J-chain** and contains a **Secretory Component**, which protects the molecule from enzymatic degradation in the harsh environments of the gut and respiratory tract. Its primary role is "mucosal immunity," preventing the attachment of pathogens to epithelial surfaces. **Why other options are incorrect:** * **IgM:** This is the largest immunoglobulin (pentamer) and the first to appear in a primary immune response. It is primarily intravascular due to its high molecular weight. * **IgG:** This is the most abundant immunoglobulin in the serum (approx. 80%). It is the only antibody that crosses the placenta but is not the predominant antibody in secretions. * **IgD:** This is primarily found on the surface of B-lymphocytes, acting as an antigen receptor. It has no significant presence in secretions. **High-Yield NEET-PG Pearls:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary infections or diarrhea. * **Breast Milk:** IgA provides passive immunity to the neonate, protecting the infant's gut. * **Half-life:** IgG has the longest half-life (approx. 23 days), while IgA has a half-life of about 6-8 days.

Question 106: Which protein motif is characteristic of steroid receptor regulatory proteins?

A. Zinc finger (Correct Answer)
B. Helix-turn-helix
C. Leucine zipper
D. RNA

Explanation: **Explanation:** The correct answer is **Zinc finger**. This question tests the understanding of DNA-binding motifs in transcription factors, a high-yield topic in both Biochemistry and Immunology. **1. Why Zinc Finger is Correct:** Steroid hormone receptors (e.g., glucocorticoid, estrogen, and progesterone receptors) belong to a superfamily of ligand-activated transcription factors. These receptors contain a specific DNA-binding domain (DBD) characterized by the **Zinc finger motif**. In this motif, a zinc ion is coordinated by cysteine and histidine residues, stabilizing a "finger-like" loop of amino acids that fits precisely into the major groove of the DNA double helix. This allows the steroid-receptor complex to bind to specific **Hormone Response Elements (HREs)** and regulate gene expression. **2. Why the other options are incorrect:** * **Helix-turn-helix:** This is the simplest and most common motif found primarily in **prokaryotic** DNA-binding proteins (e.g., the *lac* repressor) and homeodomain proteins involved in development. * **Leucine zipper:** This motif is characterized by a leucine residue at every seventh position, forming an amphipathic helix. It is typical of transcription factors like **c-Jun and c-Fos** (AP-1 complex), which function as dimers. * **RNA:** RNA is a nucleic acid, not a protein structural motif. **Clinical Pearls for NEET-PG:** * **Zinc Finger:** Associated with Steroid receptors, Thyroid hormone receptors, and Vitamin D receptors. * **Leucine Zipper:** Associated with proto-oncogenes (Jun, Fos, Myc). * **Mnemonic:** "Zinc fingers grip the Steroids" — helps remember the association between Zinc motifs and steroid/thyroid receptors.

Question 107: Complement proteins constitute what percentage of serum proteins?

A. <1%
B. 5%
C. 5-10% (Correct Answer)
D. >10%

Explanation: **Explanation:** The complement system is a vital component of innate immunity, consisting of a complex group of over 30 heat-labile proteins and glycoproteins. These proteins circulate in the blood as inactive precursors (zymogens) and are primarily synthesized in the liver. **1. Why 5-10% is correct:** Collectively, complement proteins account for approximately **5% to 10% of the total serum globulin fraction**. Among these, **C3** is the most abundant complement protein in the serum (approx. 1.2 mg/ml), playing a central role in all three activation pathways (Classical, Alternative, and Lectin). **2. Analysis of Incorrect Options:** * **<1% (Option A):** This is too low. While individual components like C1q or Factor D are present in small amounts, the cumulative percentage of all complement proteins is significantly higher. * **5% (Option B):** While 5% is the lower limit, the standard medical literature (including Harrison’s and Ananthanarayan) defines the range as 5-10%. * **>10% (Option D):** This is too high. The majority of serum proteins are comprised of Albumin (~60%) and other globulins (like Immunoglobulins). Complement proteins do not exceed these major fractions under physiological conditions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Most complement proteins are synthesized in the **liver**, except C1 (intestinal epithelium) and Factor D (adipose tissue). * **Heat Lability:** Complement is inactivated by heating serum at **56°C for 30 minutes**. * **Acute Phase Reactants:** Many complement proteins (especially C3 and C4) increase during inflammation. * **C3 Deficiency:** This is the most serious deficiency as it is the "bottleneck" of all pathways, leading to recurrent pyogenic infections. * **CH50 Assay:** Used to screen for deficiencies in the classical pathway.

Question 108: T-cells mature in:

A. Peyer's patches
B. Lymph node
C. Thymus (Correct Answer)
D. Bursa of Fabricius

Explanation: **Explanation:** The development of T-lymphocytes is a multi-stage process involving primary and secondary lymphoid organs. **1. Why Thymus is Correct:** T-cells originate from hematopoietic stem cells in the **bone marrow** but migrate to the **Thymus** for maturation and differentiation (hence the name "T" cells). In the thymus, thymocytes undergo **positive selection** (ensuring they recognize MHC molecules) and **negative selection** (eliminating self-reactive cells). This process ensures central tolerance and produces mature, immunocompetent T-cells (CD4+ and CD8+) that then enter the peripheral circulation. **2. Analysis of Incorrect Options:** * **Peyer’s Patches & Lymph Nodes:** These are **secondary lymphoid organs**. They are sites where mature lymphocytes encounter antigens and undergo activation and clonal expansion, but they are not sites of initial maturation. * **Bursa of Fabricius:** This is a primary lymphoid organ found only in **birds**, where B-cell maturation occurs. In humans, the functional equivalent for B-cell maturation is the **Bone Marrow**. **Clinical Pearls for NEET-PG:** * **DiGeorge Syndrome:** Characterized by thymic hypoplasia (3rd/4th pharyngeal pouch defect), leading to profound T-cell deficiency and recurrent viral/fungal infections. * **Hassall’s Corpuscles:** These are characteristic epithelial whorls found in the **thymic medulla**, a key histological marker. * **Involution:** The thymus is most active during childhood and undergoes fatty involution after puberty, though it continues to produce T-cells at a lower rate throughout life.

Question 109: T4/T8 ratio reversal is seen in which of the following conditions?

A. T-cell lymphoma
B. Hairy cell leukemia
C. AIDS (Correct Answer)
D. Infectious mononucleosis

Explanation: **Explanation:** The **T4/T8 ratio** (CD4+ to CD8+ ratio) is a critical marker of immune system health. In a healthy individual, the normal ratio is approximately **2:1**. A reversal (ratio <1.0) occurs when CD4+ T-helper cells are depleted or CD8+ T-cytotoxic cells are significantly increased. **1. Why AIDS is the Correct Answer:** In **AIDS (HIV infection)**, the Human Immunodeficiency Virus specifically targets and destroys **CD4+ T-cells** by binding to the CD4 receptor via its gp120 envelope protein. As the disease progresses, the absolute count of T4 cells plummet while T8 cells may initially increase or remain stable, leading to a profound **reversal of the T4/T8 ratio**. This is a hallmark laboratory finding used to monitor disease progression and the risk of opportunistic infections. **2. Analysis of Incorrect Options:** * **T-cell Lymphoma:** This is a neoplastic proliferation of T-cells. While it alters the immune profile, it typically presents with a clonal expansion of a specific T-cell subset rather than a predictable reversal of the ratio seen in HIV. * **Hairy Cell Leukemia:** This is a rare **B-cell** lymphoproliferative disorder characterized by "hairy" cytoplasmic projections. Since it involves the B-cell lineage, it does not primarily target the T4/T8 balance. * **Infectious Mononucleosis:** Caused by the Epstein-Barr Virus (EBV), this condition actually causes a massive proliferation of **CD8+ T-cells** (atypical lymphocytes or Downey cells). While this can technically lower the ratio temporarily, it is a reactive lymphocytosis rather than the diagnostic depletion-driven reversal characteristic of AIDS. **High-Yield Clinical Pearls for NEET-PG:** * **Normal CD4 count:** 500–1500 cells/mm³. * **AIDS Definition:** CD4 count **<200 cells/mm³** or the presence of an AIDS-defining illness. * **Other conditions with reversed ratio:** Measles, Cytomegalovirus (CMV) infection, and certain autoimmune diseases. * **Marker for HIV monitoring:** CD4 count is the best indicator of **immune status**, while Viral Load is the best indicator of **treatment response**.

Question 110: Which complement component(s) are necessary for Neisseria infection?

A. C5
B. C6
C. C7
D. All of the above (Correct Answer)

Explanation: The correct answer is **D. All of the above.** ### **Explanation** The complement system is a vital component of innate immunity. To eliminate Gram-negative bacteria like *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*), the body relies heavily on the **Membrane Attack Complex (MAC)**. The MAC is the final common pathway of all three complement activation routes (Classical, Alternative, and Lectin). It is composed of complement components **C5b, C6, C7, C8, and C9**. These proteins assemble on the bacterial surface to form a transmembrane pore, leading to osmotic lysis and death of the pathogen. * **C5:** The cleavage of C5 into C5b initiates the assembly of the MAC. * **C6 and C7:** These components bind sequentially to C5b to form the C5b-6-7 complex, which anchors the complex to the bacterial phospholipid bilayer. * **C8 and C9:** These complete the pore formation. Because *Neisseria* species have thin peptidoglycan layers and are particularly susceptible to direct lysis, a deficiency in **any** of these late-stage components (C5 through C9) results in a failure to form the MAC. ### **Why other options are incorrect** Options A, B, and C are individual components of the MAC. While each is necessary, they do not represent the complete requirement. Since a deficiency in any one of them leads to the same clinical susceptibility, "All of the above" is the most accurate choice. ### **High-Yield Clinical Pearls for NEET-PG** * **Late Complement Deficiency (C5–C9):** Characterized by recurrent, disseminated infections with *Neisseria* species. * **Early Complement Deficiency (C1, C2, C4):** Associated with increased risk of pyogenic infections and autoimmune diseases like **Systemic Lupus Erythematosus (SLE)**. * **C3 Deficiency:** The most severe deficiency; leads to recurrent infections with encapsulated bacteria (e.g., *S. pneumoniae, H. influenzae*) and Type III hypersensitivity reactions. * **CH50 Assay:** Used to screen for deficiencies in the classical and terminal complement pathways.

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Immunology — MCQs

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