## Cytokine Profile in Type IV Hypersensitivity **Key Point:** Interferon-gamma (IFN-γ) is the hallmark cytokine of Th1-mediated delayed-type hypersensitivity reactions. It activates macrophages and promotes their antimicrobial and cytotoxic functions. ### Mechanism of IFN-γ in DTH 1. Th1 cells recognize antigen presented on MHC Class II molecules 2. Upon activation, Th1 cells secrete IFN-γ 3. IFN-γ binds to macrophage receptors and causes: - Increased expression of MHC molecules - Enhanced antimicrobial activity - Release of inflammatory mediators (TNF-α, IL-1, IL-6) - Recruitment of additional immune cells ### Comparison of Th1 vs Th2 Cytokines | Cytokine | Source | Hypersensitivity Type | Function | |----------|--------|----------------------|----------| | IFN-γ | Th1 cells | Type IV (DTH) | Macrophage activation | | IL-4 | Th2 cells | Type I (Immediate) | B cell isotype switching to IgE | | IL-5 | Th2 cells | Type I (Immediate) | Eosinophil recruitment and activation | | TNF-α | Macrophages, T cells | Type IV (DTH) | Amplifies inflammation (secondary) | **High-Yield:** IFN-γ is the **defining cytokine** of Type IV hypersensitivity. Its presence indicates Th1 activation. This is tested frequently in NEET PG as the distinguishing feature of cell-mediated immunity. **Clinical Pearl:** In tuberculin skin test (Mantoux test), the induration at 48–72 hours is driven by IFN-γ-activated macrophages forming granulomas around the injected antigen. [cite:Robbins 10e Ch 6]
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