Clinical meaning
Rapidly progressive glomerulonephritis (RPGN) is a clinical syndrome of rapid loss of renal function (50% decline in GFR within 3 months) caused by extensive crescent formation in the glomeruli. Crescents are composed of proliferating parietal epithelial cells and infiltrating macrophages in Bowman's space, compressing the glomerular tuft and destroying the filtration barrier. RPGN is classified by immunofluorescence pattern into three types: Type I (anti-GBM disease/Goodpasture syndrome, ~10%): linear IgG deposits along the glomerular basement membrane; anti-GBM antibodies attack the collagen IV alpha-3 chain present in both glomerular and alveolar basement membranes, causing pulmonary-renal syndrome with hemoptysis and nephritis. Type II (immune complex-mediated, ~40%): granular deposits; caused by SLE, IgA nephropathy, post-infectious GN, or membranoproliferative GN. Type III (pauci-immune, ~50%): no or scant immune deposits; associated with ANCA-positive vasculitis (granulomatosis with polyangiitis [GPA/Wegener], microscopic polyangiitis [MPA], eosinophilic granulomatosis with polyangiitis [EGPA/Churg-Strauss]). The presentation is acute: oliguria, hematuria (often gross), proteinuria, hypertension, and rapidly rising creatinine over days to weeks. Treatment is urgent: high-dose IV methylprednisolone, cyclophosphamide or rituximab for vasculitis, and plasmapheresis for anti-GBM disease to remove pathogenic antibodies.