C3 and systemic lupus erythematosus: To evaluate the potential modulatory effect of reduced receptor expression, isolated B cells of healthy individuals or active SLE patients were activated via the BCR and CR1 was targeted simultaneously either by the well-characterized “C3b-like” agonist, aggregated C3 [9, 10] or the natural ligand, C3b or the CR1-specific monoclonal Ab, To5.