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Which of the following immune cell types is primarily responsible for the positive result?
Dr. Shoaib Ahmad Baig
January 15, 2025
🔍 Question
A 42-year-old male arrives at the emergency department with complaints of coughing up blood. He mentions having a persistent cough with occasional blood-streaked sputum for three months, along with fever and a weight loss of 15 lbs (6.8 kg). He has a history of intravenous drug use for the past four years and unstable housing over the last five years, frequently staying in public shelters. His vital signs are as follows: temperature of 38.6°C (101.4°F), heart rate of 94 beats per minute, respiratory rate of 17 breaths per minute, blood pressure of 102/74 mmHg, and oxygen saturation of 95% on room air. Physical examination reveals coarse breath sounds in the left lower lung and dried blood around his mouth. An interferon-gamma release assay is performed, which shows a positive result.
Which of the following immune cell types is primarily responsible for the positive result?
• A. Macrophages
• B. Neutrophils
• C. Plasma cells
• D. Red blood cells
• E. T lymphocytes
Correct Answer 🎯: E. T lymphocytes
⚙️
🧠 Trigger: Infection with Mycobacterium tuberculosis
↓
Inhalation of aerosolized bacteria
⬇
Alveolar macrophages phagocytose bacteria
⬇
Incomplete bacterial killing (TB evades phagolysosome fusion)
⬇
Bacteria replicate inside macrophages
⬇
Macrophages release pro-inflammatory cytokines (e.g., IL-12)
⬇
T lymphocytes activation via antigen presentation (MHC-II)
⬇
🧠 Key Insight: T lymphocytes secrete interferon-gamma (IFN-γ)
⬆ IFN-γ ↑ stimulates macrophage activation
⬇
Formation of granulomas (to contain infection)
⬇
Granuloma necrosis → Caseous necrosis
⬇
Erosion into nearby vessels or bronchi
⬇
Hemoptysis and productive cough
Symptoms Explained 🔧
Fever:
Cytokines release (e.g., TNF-α, IL-1) ⬇
Hypothalamic temperature set-point ↑
Weight Loss:
Cytokines (e.g., TNF-α) ⬇
Hypermetabolic state + appetite suppressionCoarse breath sounds:
Granuloma formation in lung parenchyma ⬇
Disrupted air flow
Hemoptysis:
Granuloma necrosis → Vessel erosion ⬇
Bleeding into airways
⚖️ Explanation of Other Differentials
Differential ⚖️ | Why Incorrect? 🔴 |
|---|---|
Macrophages | ⚙ Initiate the immune response but rely on T lymphocytes for IFN-γ production and granuloma maturation. |
Neutrophils | Acute-phase responders; not involved in chronic TB pathophysiology or IGRA response. |
Plasma Cells | Antibody producers; irrelevant to cell-mediated immunity assessed by IGRA. |
Red Blood Cells | Oxygen carriers; play no role in immunity or IGRA response. |
📝 Flashcards
Stay Hungry, Stay Curious!
Your Brother In This Struggle
Dr. Shoaib Ahmad
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