## Immunofluorescence Pattern in PSGN **Key Point:** Post-streptococcal glomerulonephritis (PSGN) shows a characteristic **'starry sky'** appearance on immunofluorescence microscopy with IgG, IgM, and C3 deposited in a **granular pattern** along the glomerular basement membrane and in the mesangium. ### Pathophysiology PSGN is an immune complex–mediated glomerulonephritis that occurs 1–3 weeks after group A streptococcal pharyngitis or skin infection. The streptococcal antigens form immune complexes with host antibodies, which deposit in the glomerulus and activate complement via the classical pathway. ### Immunofluorescence Findings | Feature | PSGN | |---------|------| | **Pattern** | Granular (lumpy-bumpy) | | **Immunoglobulins** | IgG, IgM, IgA (variable) | | **Complement** | C3 (prominent), C1q, C4 | | **Distribution** | Subepithelial (humps), subendothelial, mesangial | | **Electron microscopy** | Subepithelial electron-dense deposits (humps) | **High-Yield:** The **'starry sky'** pattern (bright granular IgG + IgM + C3) is pathognomonic for PSGN and distinguishes it from IgA nephropathy (mesangial IgA) and lupus (linear IgG + IgM + IgA + C1q + C4). ### Clinical Correlation **Clinical Pearl:** PSGN typically presents with acute nephritic syndrome (hematuria, proteinuria, hypertension, edema) in children 2–6 weeks after streptococcal infection. Most cases resolve spontaneously; only 1–2% progress to chronic kidney disease in children. **Mnemonic: PSGN Immune Complex Pattern — "GIM Granular"** - **G** = Granular pattern - **I** = IgG, IgM (and C3) - **M** = Mesangial + subepithelial humps
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