Direct immunofluorescence shows mesangial and capillary loop granular fluorescence. Cases of idiopathic membranoproliferative glomerulonephritis usually show positivity for IgG and C3. Membranoproliferative glomerulonephritis, type I is a morphologic diagnosis, not an etiologic diagnosis. Cases of lupus glomerulonephritis and cryoglobulinemic glomerulonephritis frequently show a membranoproliferative, type I pattern of injury. I is more a morphologic pattern of injury that a single disease.
Many cases are idiopathic.
I is an immune complex mediated disorder. Immune complexes are deposited in the mesangium and subendothelial spaces. Mesangial cells proliferate in response to the immune complex deposition and extend beyond the confines of the mesangium to mesangialize the capillary loops. A second internal basement membrane is formed by the mesangial cell and the undermined endothelial cells. Idiopathic membranoproliferative glomerulonephritis type I tend to be seen in older children and in adults.