## Immunological Basis of Type 1 Lepra Reaction **Key Point:** Type 1 lepra reaction (reversal reaction) is a **delayed-type hypersensitivity reaction (Type IV)** mediated by **Th1 cells and cellular immunity**, not antibodies. ### Mechanism 1. **Cell-mediated immunity activation** — Th1 lymphocytes recognize *Mycobacterium leprae* antigens presented on HLA-DR molecules 2. **Cytokine release** — IFN-γ and TNF-α from activated T cells drive macrophage activation 3. **Granuloma formation** — Enhanced granulomatous inflammation at existing lesion sites 4. **Clinical onset** — Typically occurs during treatment (within first 6 months) or at borderline forms of disease ### Distinguishing Type 1 from Type 2 Reactions | Feature | Type 1 (Reversal) | Type 2 (ENL) | |---------|-------------------|---------------| | **Hypersensitivity type** | Type IV (cellular) | Type III (immune complex) | | **Immune mechanism** | Th1 cell activation | Immune complex deposition | | **Timing** | During/after treatment | Usually after 2+ years of disease | | **Lesion distribution** | Existing lesions inflame | Systemic manifestations | | **Bacillary load** | LL/BL forms (high load) | LL forms (very high load) | **High-Yield:** Type 1 = **Th1-mediated** (remember: "**1**-cell-mediated"), Type 2 = **immune complex** (remember: "**2** antigens + antibodies"). **Clinical Pearl:** Type 1 reactions occur in the **borderline spectrum** (BT, BL, BB) and reflect upgrading or downgrading of immunity; they are a sign of **immune reconstitution** and often indicate the patient is responding to treatment. [cite:Park 26e Ch 12] 
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