Immunology Practice Questions

Q: Complement Fixation test is:

Wassermann reaction. **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.

Q: Which RNA polymerase transcribes microRNA?

RNA polymerase II. **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.

Q: Which of the following is NOT a live vaccine?

Salk vaccine. 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).

Q: Which immunoglobulin is found predominantly in secretions?

IgA. **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.

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

Zinc finger. **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.

Q: Complement proteins constitute what percentage of serum proteins?

5-10%. **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:** * ** 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.

Q: T-cells mature in:

Thymus. **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 1

Complement Fixation test is:

Accepted Answer

Wassermann reaction

Answer

WIDAL

Answer

Coombs test

Answer

VDRL

Question 2

Which RNA polymerase transcribes microRNA?

Accepted Answer

RNA polymerase II

Answer

RNA polymerase I

Answer

RNA polymerase III

Answer

DNA polymerase

Question 3

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

Accepted Answer

Goodpasture syndrome

Answer

Systemic Lupus Erythematosus (SLE)

Answer

Autoimmune Hemolytic Anemia (AIHA) reaction

Answer

Post-streptococcal glomerulonephritis

Question 4

Which of the following is NOT a live vaccine?

Accepted Answer

Salk vaccine

Answer

BCG

Answer

Measles

Answer

Mumps

Question 5

Which immunoglobulin is found predominantly in secretions?

Accepted Answer

IgA

Answer

IgM

Answer

IgG

Answer

IgD

Question 6

Which protein motif is characteristic of steroid receptor regulatory proteins?

Accepted Answer

Zinc finger

Answer

Helix-turn-helix

Answer

Leucine zipper

Answer

RNA

Question 7

Complement proteins constitute what percentage of serum proteins?

Accepted Answer

5-10%

Answer

<1%

Answer

5%

Answer

>10%

Question 8

T-cells mature in:

Accepted Answer

Thymus

Answer

Peyer's patches

Answer

Lymph node

Answer

Bursa of Fabricius

Question 9

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

Accepted Answer

AIDS

Answer

T-cell lymphoma

Answer

Hairy cell leukemia

Answer

Infectious mononucleosis

Question 10

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

Accepted Answer

All of the above

Answer

C5

Answer

C6

Answer

C7

Immunology — MCQs

Immunology — MCQs

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