Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Correctly match the type of Hypersensitivity reaction with its respective
Example.
Hypersensitivity reaction Example
P. Type I 1. Tuberculin reaction
Q. Type II 2. Arthus reaction
R. Type III 3. Chronic urticaria
S. Type IV 4. Systemic anaphylaxis
Options:
(A) P-3; Q-4; R-2; S-1
(B) P-4; Q-3; R-1; S-2
(C) P-4; Q-3; R-2; S-1
(D) P-2; Q-3; R-4; S-1
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.39)**
**1. Governing Concept**:
- **Topic**: Immunology & Heavy Chain Class Switching (Isotype Switching)
- Isotype switching changes the constant region ($C_H$) of the immunoglobulin heavy chain while preserving the antigen-binding variable region ($V_H/V_L$):
- Mediated by the enzyme **Activation-Induced Cytidine Deaminase (AID)**, which deaminates cytosines to uracils in switch ($S$) regions upstream of constant region genes.
- Recombination between switch regions (e.g. $S\mu$ and $S\gamma$ or $S\epsilon$) deletes intervening DNA, bringing the $V(D)J$ segment adjacent to a new heavy chain constant gene ($C_\gamma, C_\alpha, \text{ or } C_\epsilon$).
- Changes the antibody class (from IgM to IgG, IgA, or IgE) and effector function, but **antigen specificity remains unchanged**.
**2. Option Analysis**:
- **Option (A) is CORRECT**: The antigen-binding specificity and idiotype of the antibody remain completely unaltered because the $V_H$ and $V_L$ variable domains are preserved.
- **Option (B) is INCORRECT**: Affinity maturation occurs through somatic hypermutation, not class switching.
- **Option (C) is INCORRECT**: Light chains do not undergo class switching.
- **Option (D) is INCORRECT**: Class switching is an irreversible DNA deletion event, not reversible alternative RNA splicing (except for membrane vs. secreted IgM/IgD).
**3. Final Answer**:
- **Correct Choice**: **A**
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