C3 and systemic lupus erythematosus: Using SLE as a response, multiple logistic regression analysis of plasma protein data suggested that reduced levels of C3, C4, and ACLA-IgG, and female gender were strong risk factors for SLE C3 is downstream of C4 in the classical and the MBL activation pathways, the activation and consumption of C3 are amplified by a positive feedback mechanism (79–81).