A 38-year-old man with borderline lepromatous leprosy on multidrug therapy for 6 months presents with acute onset fever, painful nodules on extensor surfaces, and neuritis. Laboratory findings show elevated ESR and CRP. All of the following are features of Erythema Nodosum Leprosum (ENL) EXCEPT:

Explanation

## Immunopathology of ENL **Key Point:** ENL (Erythema Nodosum Leprosum) is fundamentally a Type III hypersensitivity reaction (immune complex-mediated), NOT a Type IV (delayed-type) reaction. ### Type III vs Type IV in Lepra Reactions | Feature | ENL (Type III) | Lepra Reaction Type 1 (Type IV) | |---------|---|---| | **Mechanism** | Immune complex deposition | Delayed-type hypersensitivity (T-cell mediated) | | **Antigen source** | Mycobacterial antigens from dead bacilli | Mycobacterial antigens in lesions | | **Timing** | During/after treatment (bacillary load decreasing) | Can occur before, during, or after treatment | | **Lesion pattern** | Nodules, erythema, systemic symptoms | Inflammation of existing lesions | | **Histology** | Vasculitis, neutrophilic infiltration | Epithelioid granulomas | ### Clinical Features of ENL (All Correct) **High-Yield:** ENL occurs in **lepromatous (LL) and borderline lepromatous (BL)** forms only—never in tuberculoid disease. 1. **Type III hypersensitivity** — immune complexes deposit in blood vessel walls, skin, and organs 2. **Timing** — typically within 2 years of starting MDT (can occur up to 10 years later) 3. **Clinical presentation:** - Painful nodules (erythema nodosum-like) - Fever, malaise - Neuritis (painful nerves) - Iritis, episcleritis - Lymphadenopathy 4. **Systemic involvement** — hepatitis, nephritis, orchitis possible ### Why Option 4 is Wrong **Warning:** Do NOT confuse ENL with Lepra Reaction Type 1 (reversal reaction). - **Lepra Reaction Type 1** = Type IV hypersensitivity (delayed-type, T-cell mediated) - **ENL** = Type III hypersensitivity (immune complex-mediated) ENL is caused by **circulating immune complexes** formed against mycobacterial antigens, not by direct T-cell recognition of antigen in lesions. **Clinical Pearl:** The presence of systemic symptoms (fever, malaise) and vasculitis in ENL reflects the systemic nature of immune complex disease, whereas Type 1 reactions are primarily local inflammatory responses in existing lesions. [cite:Park 26e Ch 18]