The cells that proliferate are mesangial cells and endothelial cells. Hypercellularity is also contributed to by neutrophil infiltration. It is in response to cellular injury mediated by the deposition of antigen-antibody complexes in the mesangium and on the basement membrane. Proteinuria is typically seen. The involvement here of some capillary loops signifies segmental lesions.
Thickening is due to deposition of immune complexes on the basement membrane.
In SLE, immune complexes formed in circulation or in situ are deposited along the basement membrane. These appear as discrete humps. By contrast, in Goodpasture's disease the antibodies are directed against the basement membrane, and hence they deposit all along its surface. These then produce a linear pattern. SLE is an immune complex disease, in which immune complexes deposited in tissues activate complement.