In a large renal pathology series from a tertiary centre in India, which is the most common site of immune complex deposition in post-streptococcal glomerulonephritis on immunofluorescence microscopy?

Explanation

## Immune Complex Deposition Sites in PSGN **Key Point:** In post-streptococcal glomerulonephritis (PSGN), the most characteristic and most commonly described site of immune complex deposition on immunofluorescence and electron microscopy is the **subepithelial** location, producing the classic "humps" — large, dome-shaped, electron-dense deposits on the outer aspect of the glomerular basement membrane (GBM). ### Electron Microscopy Findings in PSGN | Deposit Location | EM Appearance | Clinical Significance | |------------------|---------------|----------------------| | **Subepithelial** | "Humps" — large, dome-shaped, electron-dense deposits | **Most characteristic and most common site**; pathognomonic for PSGN | | **Subendothelial** | Electron-dense, granular; between endothelium and GBM | Present but less prominent; contributes to endocapillary proliferation | | **Mesangial** | Deposits in mesangial matrix | Mild mesangial proliferation; secondary finding | | **Intramembranous** | Deposits within GBM substance | Rare in PSGN; more typical of membranoproliferative GN | ### Immunofluorescence Pattern in PSGN On immunofluorescence, PSGN shows a **granular ("starry sky") pattern** of IgG and C3 along the capillary walls, corresponding predominantly to the subepithelial humps. This granular capillary wall pattern is the hallmark IF finding. **High-Yield (Robbins Pathologic Basis of Disease, 10th ed.):** The subepithelial "humps" are described as the **most characteristic** deposits in PSGN. They are large, discrete, electron-dense deposits on the epithelial side of the GBM. While subendothelial deposits may also be present, the subepithelial humps are the defining and predominant finding that distinguishes PSGN from other proliferative GNs. **Clinical Pearl:** The "humps" (subepithelial deposits) are both **pathognomonic** AND the **most common/characteristic** site of deposition in PSGN. They typically resolve within 6–8 weeks as the disease resolves, correlating with the excellent prognosis of PSGN in children. Subendothelial deposits, while present, are not the predominant or defining feature. **Warning:** A common exam trap is to confuse "subendothelial" (characteristic of lupus nephritis class III/IV and membranoproliferative GN) with PSGN. In PSGN, the subepithelial hump is the hallmark deposit. Do not conflate the mechanism of endocapillary proliferation (driven partly by complement activation) with the primary deposit site. *Reference: Robbins & Cotran Pathologic Basis of Disease, 10th edition, Chapter on Glomerular Diseases; Harrison's Principles of Internal Medicine, 21st edition.*