Figure 1: The complement system and its regulators. The complement system has four main steps. (A) Classical, alternative, or lectin pathway activation produces C3 convertases (C3bBb or C4bC2a) to initiate the complement cascade. (B) C3 convertase cleaves C3 into C3a and C3b. CFB binds to C3b and is cleaved by CFD into Bb, forming a new C3 convertase, C3bBb. This amplification step is tightly controlled by multiple regulators of complement (e.g., CFH, MCP, DAF, and CFI). (C) Once C3 convertase amplification is allowed to proceed, additional C3b is generated, ultimately forming C5 convertase, C3BbC3b. (D) C5 convertase cleaves C5 into C5b, which recruits C6, 7, 8, and 9 to form the membrane attack complex.