A farmer presents with erythema and induration at the site of a tuberculin (Mantoux) test 48-72 hours after injection. This delayed-type hypersensitivity reaction is mediated by:
- A IgE-sensitized mast cells releasing histamine
- B Antigen-specific CD4+ Th1 cells releasing IFN-γ and activating macrophages ✓
- C Complement-mediated neutrophil recruitment
- D Immune complex deposition with complement activation
Correct answer: B. Antigen-specific CD4+ Th1 cells releasing IFN-γ and activating macrophages
Explanation
Type IV (delayed-type) hypersensitivity is mediated by antigen-specific CD4+ Th1 lymphocytes sensitized by prior exposure to the antigen. On re-exposure, these cells release cytokines such as IFN-γ, TNF-β, and IL-2, which recruit and activate macrophages, causing the characteristic induration and erythema peaking at 48-72 hours. It requires no antibody or complement. The Mantoux test for tuberculosis, contact dermatitis, and granuloma formation in tuberculosis are classic examples.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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