Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
An immunocompetent person becomes infected with a pathogenic strain of bacteria. The time course of bacterial clearance by adaptive immune response typically shows:
Options:
(A) Initial exponential rise in bacteria followed by rapid clearance as specific IgG/IgM titers peak
(B) Continuous exponential growth of bacteria leading to fatal load
(C) Immediate zero growth from time t = 0 due to passive resistance
(D) Oscillatory steady state that never reaches clearance
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.40)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Immunology & Infectious Disease Kinetics (Bacterial Load & Adaptive Immunity Clearance)
**2. Detailed Step-by-Step Analysis**:
- When an immunocompetent individual is infected with a pathogenic bacterial strain:
1. **Initial Phase (Infection & Incubation)**: The bacteria multiply within host tissues, causing the bacterial burden/load to increase exponentially over the first few days as innate immunity delays but cannot fully clear the pathogen.
2. **Induction of Adaptive Immunity**: Around days 5–7, antigen-presenting cells successfully prime naive T and B lymphocytes in secondary lymphoid organs, resulting in clonal expansion and differentiation of effector $CD4^+$ T cells, $CD8^+$ cytotoxic T cells, and antibody-secreting plasma cells.
3. **Clearance Phase**: High-affinity antibodies (opsonization and complement-mediated lysis) together with fully activated macrophages clear the pathogen, driving the bacterial load down toward zero (sterilizing immunity).
- Graphically, plotting bacterial load versus time yields a curve that **rises to a distinct peak and then drops steeply to baseline zero** (Curve B).
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Shows continuous uncontrolled exponential bacterial growth, which occurs in severe combined immunodeficiency (SCID) or fatal bacteremia, not an immunocompetent person.
- **Option (B) [CORRECT]**: Accurately shows initial bacterial replication followed by complete immunological clearance driven by the adaptive immune response.
- **Option (C) [INCORRECT]**: Shows immediate linear clearance without any microbial multiplication phase.
- **Option (D) [INCORRECT]**: Shows a persistent plateau (chronic carrier state without clearance).
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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