Autoantibodies to C1q, which bind to epitopes on the collagen-like region (CLR) of the C1q molecule, are one of the best biomarkers of active, proliferative forms of lupus nephritis. Anti-C1q antibodies probably arise after immunization with neoantigens on C1q bound to immune complexes or to early apoptotic cells, and they play a pathogenic role in augmenting immune complex glomerulonephritis by leading to aggregation of glomerular basement membrane immune complexes that have bound C1q.