Subtopic: Fundamentals of Microbial growth kinetics
Question Statement:
Which of the following statements about immune response are CORRECT?
[P] T cells are activated by antigen-presenting cells
[Q] Foreign peptides are not presented to helper T cells by Class II MHC proteins
[R] Dendritic cells are referred to as professional antigen-presenting cells
Options:
(A) P and R only
(B) P and Q only
8 ---
(C) Q and R only
(D) P, Q and R
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.38)**
**1. Governing Concept**:
- **Topic**: Immunology & Primary vs. Secondary Immune Responses
- Comparing humoral immune kinetics upon primary vs. secondary antigenic challenge:
- **Primary response**: Lag phase is long ($\sim 7-10\text{ days}$); initial antibody isotype secreted is low-affinity **IgM**, followed by moderate levels of IgG.
- **Secondary (anamnestic) response**: Due to the presence of antigen-specific memory B and T cells, the lag phase is significantly shorter ($1-3\text{ days}$), the rate of antibody production is faster, and the predominant antibody is high-affinity **IgG** produced in massive titers (affinity maturation via somatic hypermutation).
**2. Option Analysis**:
- **Option (A) is CORRECT**: Secondary immune response exhibits shorter lag phase, higher affinity antibodies, and higher IgG titers compared to primary response.
- **Option (B) is INCORRECT**: Secondary response produces predominantly IgG, not IgM.
- **Option (C) is INCORRECT**: Primary response is slower, not faster, than secondary.
- **Option (D) is INCORRECT**: Antibody affinity in secondary response is higher, not lower.
**3. Final Answer**:
- **Correct Choice**: **A**
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