Decay-accelerating-factor (DAF, CD55) is a membrane- bound complement regulatory protein that inhibits autologous complement cascade activation. It is expressed on all cells that are in close contact with serum complement proteins, but also on cells outside the vascular space and on tumour cells. DAF binds to C3bBb and C4bC2a on cell surfaces, accelerating their dissociation and thereby inhibiting the amplification of complement. DAF can bind C3b and Bb, and must bind both for efficient decay acceleration. Although it can bind the inactive proenzymes C3b and C4b, the regulatory function of DAF is believed to be inhibition of activated C3 convertase enzymes (Harris et al. 2007).