Skip to main content
. 2018 Apr 11;9:735. doi: 10.3389/fimmu.2018.00735

Figure 2.

Figure 2

Overview of complement factor I (CFI) function. (A) CFI inhibits both the classical and lectin complement pathways by deactivation of C4bC2a complex. Surface-bound C4bC2a activating complex is bound by C4b-binding protein (C4BP) leading to the dissociation of C2a. CFI subsequently binds to the C4b-C4BP complex leading to the cleavage of C4b into C4c and C4d. (B) CFI deactivates complement receptor 1 (CR1)/CD35-bound C3b. Membrane cofactor protein (MCP/C46) binds to C3b-CR1 complex. The association of CFI results in the cleavage of C3b. (C) CFI inhibits the alternative complement pathway by deactivation of C3b: surface-bound C3b is cleaved into iC3B and C3d by factor H and CFI. Thus, whilst CFI is a major regulator of both the classical and alternative complement pathways, the alternative pathway is most affected, hence C3 is typically much lower than C4 in cases of CFI deficiency (see main text). Furthermore, since factor H is an important regulator of the alternative pathway, secondary factor H deficiency is observed in CFI deficiency (see main text).