A. C1. C1 inhibitor (C1 INH) acts in 2 ways. It binds free C1 in serum ( [C1 INH] > [C1] ) and inhibits spontaneous activation of C1; it is released on activation by immune complexes. It also limits the activation of C4 and C2 by binding and inhibiting the C1r and C1s proteases.

B. C3 convertases. The lifetime of C3 convertases is limited by 2 distinct but related means. The lifetime of the complexes is reduced by decay accelerating factors. Some of these are present on host cell surfaces such as Decay Accelerating Factor (DAF) and CR1, which act on both the classical and alternative C3 convertases. Others are serum proteins, C4 binding protein (C4bp) and factor H act respectively on C4b2b and C3bBb. All these molecules promote the dissociation of the C3 convertases by binding to the covalently bound component (C4b or C3b) displacing the associated cofactor. In addition C4bp, fH and CR1 and another protein (MCP) catalyse the permanent inactivation of C3b and C4b via protealytic cleavage by factor I (generating iC3b etc.).

Added Value
Factor I continues to cleave iC3b and leaves the small, stable C3d fragment bound to the immune complex or pathogen surface via the covalent thiol-ester bond.