Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Macrophages eliminate pathogenic bacteria upon activation by
Options:
(A) NK cells
(B) basophils
(C) CD4+ T cells
(D) plasma cells
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.13)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Immunology (Cell-Mediated Immunity & Macrophage Activation)
**2. Detailed Step-by-Step Analysis**:
- Macrophages are tissue-resident phagocytes that ingest pathogenic bacteria into phagosomes. However, many intracellular bacteria (such as *Mycobacterium tuberculosis*, *Listeria monocytogenes*, and *Leishmania*) can resist killing inside resting macrophages.
- Full microbicidal activation of macrophages (the "classical activation" or M1 phenotype) requires assistance from antigen-specific **$CD4^+$ T cells** (specifically, **$T_H1$ cells**):
1. The macrophage presents degraded bacterial peptides on **MHC Class II** molecules to the T-cell receptor (TCR) of a $CD4^+$ $T_H1$ cell.
2. The activated $CD4^+$ T cell secretes **Interferon-$\gamma$ (IFN-$\gamma$)** and engages **CD40** on the macrophage via **CD40L**.
3. This dual signaling activates the macrophage to transcribe inducible nitric oxide synthase (iNOS), produce respiratory burst reactive oxygen species (ROS), and induce lysosomal fusion, leading to the destruction of the internalized pathogens.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: NK cells participate in innate cytotoxicity against viral/tumor cells, but classical pathogen elimination via cell-mediated adaptive immunity is coordinated by $CD4^+$ T helper cells.
- **Option (B) [INCORRECT]**: Basophils are granulocytes mediating allergic responses and antiparasitic defenses.
- **Option (C) [CORRECT]**: $CD4^+$ T cells ($T_H1$) secrete IFN-$\gamma$ and express CD40L to activate macrophages into efficient microbicidal effectors.
- **Option (D) [INCORRECT]**: Plasma cells are terminally differentiated B cells that secrete soluble antibodies, not activating macrophages directly.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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