Which of the following cytokines is the primary mediator of the delayed-type hypersensitivity (Type IV) response?

Explanation

## Type IV Hypersensitivity Cytokine Cascade **Key Point:** Type IV hypersensitivity (cell-mediated immunity) is orchestrated by T-helper 1 (Th1) cells, which secrete IFN-γ as their signature cytokine. ### Role of IFN-γ IFN-γ is the **primary mediator** of delayed-type hypersensitivity because it: 1. Activates macrophages to become cytotoxic and secrete pro-inflammatory mediators 2. Upregulates MHC class II expression on antigen-presenting cells 3. Promotes recruitment and retention of T cells at the site of antigen challenge 4. Induces production of other inflammatory cytokines (IL-12, TNF-α) ### Comparison of Cytokines in Hypersensitivity | Cytokine | Source | Hypersensitivity Type | Function | |----------|--------|----------------------|----------| | IFN-γ | Th1 cells | Type IV (DTH) | Macrophage activation, cell-mediated immunity | | IL-4 | Th2 cells | Type I (IgE-mediated) | B cell class switching to IgE | | IL-5 | Th2 cells | Type I (IgE-mediated) | Eosinophil recruitment and activation | | TNF-α | Macrophages, mast cells | Type I, II, III, IV | General inflammation (amplifier, not primary) | **High-Yield:** IFN-γ is the **hallmark cytokine of Type IV hypersensitivity**. Tuberculin skin test (Mantoux test) positivity is driven by IFN-γ-producing memory T cells. **Clinical Pearl:** Patients with IFN-γ receptor deficiency cannot mount effective Type IV responses and are susceptible to intracellular pathogens (mycobacteria, salmonella). **Mnemonic:** **Th1 = IFN-γ = Type IV**. Remember: Th1 (one) → IFN-γ → Type IV (cell-mediated).