Decay-accelerating factor

Complement decay-accelerating factor, also known as CD55 or DAF, is a protein that, in humans, is encoded by the CD55 gene.

Decay accelerating factor is a 70 kDa membrane protein that regulates the complement system on the cell surface. DAF recognizes C4b and C3b fragments that are created during C4 (classical complement pathway and lectin pathway) and C3 (alternate complement pathway) activation. Interaction of DAF with cell-associated C4b of the classical and lectin pathways interferes with the conversion of C2 to C2a, thereby preventing formation of the C4b2a C3 convertase, and interaction of DAF with C3b of the alternative pathway interferes with the conversion of factor B to Bb by factor D, thereby preventing formation of the C3bBb C3 convertase of the alternative pathway. Thus, by limiting the amplification convertases of the complement cascade, DAF indirectly blocks the formation of the membrane attack complex.