Mild to moderate mesangial hypercellularity is typically seen using light microscopy. A subset of cases feature endocapillary hypercellularity, which can be segmental or global, focal or diffuse, and with or without sclerosing lesions. Proliferation and sclerosis often coexist in the same biopsy, which suggests that proliferative lesions lead to scarring and sclerosis. Extracapillary cellular crescents (see Plate 4-25) may occur if there is diffuse endocapillary hypercellularity, but these rarely involve more than 50% of glomeruli. IgA immune deposits may be seen in the mesangium and, in cases with endocapillary proliferation, the subendothelium.

The degree of tubular atrophy and interstitial fibrosis generally reflects the degree of glomerular scarring. Tubules may contain intraluminal RBCs. In cases of severe hematuria, the RBCs may cause tubular obstruction with subsequent tubular injury. In cases with long-standing hematuria, tubules may show hemosiderin granules. If there is hypertension, arterial vessels typically show a proportionate degree of mild to moderate medial sclerosis and intimal fibrosis. True vasculitis, however, is rare and suggests the alternative diagnosis of Henoch-Schönlein purpura.