Figure 1. Simplified overview of the complement cascade. The complement cascade is comprised of more than 30 proteins. The cascade can be activated through three distinct pathways. C3 and C4 are routinely measured in patients suspected of having glomerular disease. C3 is the central component of the cascade, and activation of all three pathways can lead to C3 cleavage. Activation of the classical and mannose binding lectin pathway causes cleavage of C4. Activation through all three pathways generates the same downstream pro-inflammatory fragments: C3a, C5a, C3b, and C5b-9.