In acute post-streptococcal glomerulonephritis, which immunofluorescence pattern is pathognomonic?

Explanation

## Immunofluorescence in Post-Streptococcal GN ### Pathognomonic Pattern **Key Point:** The **'starry sky' pattern** — C3-dominant granular deposits with IgG and IgM — is the classic immunofluorescence finding in acute post-streptococcal glomerulonephritis (APSGN). ### Why C3-Dominant? **High-Yield:** APSGN is mediated by **classical complement pathway activation**. The immune complexes (GAS antigen + antibody) activate C1q, leading to: 1. Consumption of C3 (hence C3 deposition) 2. Relative sparing of C4 (C4 levels often normal or mildly reduced) 3. Granular, subepithelial 'hump' deposits on electron microscopy ### Immunofluorescence Comparison Table | Disease | IF Pattern | Pathophysiology | Complement Profile | | --- | --- | --- | --- | | APSGN | C3-dominant + IgG/IgM ('starry sky') | Classical pathway | C3↓, C4 normal | | IgA Nephropathy | IgA-dominant, mesangial | IgA immune complex | Normal complement | | Lupus GN | IgG + IgA + IgM + C3 + C4 ('full house') | Immune complex, classical pathway | C3↓, C4↓ | | ANCA-GN | Minimal/absent IF | ANCA-mediated, pauci-immune | Normal complement | | Linear IgG (Anti-GBM) | Linear IgG along GBM | Autoantibody to α3(IV) | Normal complement | **Clinical Pearl:** The 'starry sky' appearance on IF corresponds to the **subepithelial 'humps'** seen on electron microscopy — these are pathognomonic for APSGN. **Mnemonic:** **C3-STARRY** = C3-dominant Subepithelial Deposits Are Recognizable in Rapid-onset glomerulonephritis from Rheumatic streptococcal infection.