## IgA Vasculitis: Pathophysiology and Clinical Features **Key Point:** IgA vasculitis is a small-vessel vasculitis characterized by IgA-dominant immune complex deposition in skin, kidneys, joints, and GI tract. ### Diagnostic Hallmark IgA immune complex deposition on immunofluorescence microscopy of affected tissues (skin or kidney biopsy) is the gold standard for diagnosis. ### Classic Clinical Tetrad 1. Palpable purpura (typically lower extremities and buttocks) 2. Arthritis/arthralgia (knees and ankles) 3. Abdominal pain (GI involvement) 4. Glomerulonephritis (IgA nephropathy on biopsy) **High-Yield:** Not all four features need be present simultaneously; purpura + one other manifestation is sufficient for diagnosis. ### Pathophysiology IgA1-dominant immune complexes deposit in small vessels, activating complement via the alternative pathway. This leads to vasculitis affecting multiple organ systems. **Clinical Pearl:** IgA vasculitis is the most common systemic vasculitis worldwide and the most common primary glomerulonephritis in Asia and Europe. ### Differentiation from Other Vasculitides | Feature | IgA Vasculitis | GPA | PAN | Takayasu | |---------|---|---|---|---| | **Vessel size** | Small | Small-medium | Medium | Large | | **Immune complex** | IgA-dominant | ANCA-associated | Immune complex | Non-immune | | **Kidney involvement** | IgA nephropathy | Pauci-immune GN | Rare | Rare | | **Skin manifestation** | Palpable purpura | Nodules, ulcers | Nodules | Erythema nodosum | | **GI involvement** | Common | Rare | Common | Rare | [cite:Harrison 21e Ch 319]
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