Proliferative Glomerulonephritis: Immune Deposition and Glomerular Cell Proliferation

Core Idea

Proliferative GN features subendothelial or subepithelial immune complex deposition activating complement and recruiting leukocytes. Proliferation of glomerular endothelial and mesangial cells with crescent formation can occlude Bowman space, causing acute kidney injury and progressive loss of GFR if untreated.

Explainer

From your study of glomerulonephritis types, you know the basic clinical distinction: nephritic syndromes feature hematuria, hypertension, and inflammation, while nephrotic syndromes feature massive protein loss. Proliferative GN is the archetypal nephritic disease, and understanding it requires connecting your knowledge of immune complex pathology to the specific anatomy of the glomerulus.

The glomerulus is a specialized capillary bed enclosed in Bowman's capsule. Its filtration surface has three layers: the fenestrated endothelium lining the capillary lumen, the glomerular basement membrane (GBM), and the podocyte layer with its interdigitating foot processes on the urinary side. In proliferative GN, immune complexes — antibody-antigen aggregates — deposit in specific anatomical locations that determine the disease phenotype. Subendothelial deposits (between endothelium and GBM, as in diffuse proliferative lupus nephritis) sit inside the capillary wall, accessible to circulating complement and leukocytes, producing aggressive inflammation. Subepithelial deposits (between GBM and podocytes, as in membranous nephropathy) sit outside the capillary, shielded from leukocytes, producing slower proteinuria without overt inflammation. Mesangial deposits (in the central supporting matrix, as in IgA nephropathy) cause mesangial cell activation and expansion. Location predicts behavior.

The inflammatory cascade follows directly from complement and leukocyte recruitment. Immune complex deposition activates the classical complement pathway, generating C3a and C5a — potent chemoattractants that summon neutrophils and monocytes into the glomerular capillaries. These leukocytes release proteases and reactive oxygen species that damage the GBM and disrupt the filtration barrier, producing hematuria (red cells squeezed through damaged capillary walls) and modest proteinuria. Meanwhile, the resident glomerular cells — endothelial cells, mesangial cells, and sometimes parietal epithelial cells lining Bowman's capsule — proliferate in response to growth factors and cytokines. This proliferation is what gives the disease its name and its characteristic histological appearance: a glomerulus packed with cells rather than the normal sparse population.

The most severe consequence is crescent formation: structures filling Bowman's space built from proliferating parietal epithelial cells and infiltrating macrophages, scaffolded by fibrin that leaks in when GBM ruptures. Glomeruli with crescents are functionally obliterated — they cannot filter. When >50% of glomeruli contain crescents, the condition is classified as rapidly progressive GN (RPGN), and GFR can fall catastrophically over days to weeks. This is a renal emergency: untreated RPGN frequently leads to dialysis dependence within weeks, which is why aggressive immunosuppression is initiated even before complete biopsy results return. The speed of crescent formation makes RPGN one of the few conditions in nephrology where treatment delay is directly measured in kidney function lost.