Decay accelerating factor (DAF, CD55) is a widely distributed membrane protein. It accelerates the dissociation of C3bBb and C4C2a, thereby inhibiting the amplification of complement. DAF can bind C3b and Bb but must bind both for efficient decay acceleration. The regulatory function of DAF is believed to be inhibition of activated C3 convertase enzymes rather than binding of inactive proenzymes (Harris et al. 2007).