Function: This protein recognizes C4b and C3b fragments that condense with cell-surface hydroxyl or amino groups when nascent C4b and C3b are locally generated during C4 and c3 activation. Interaction of daf with cell-associated C4b and C3b polypeptides interferes with their ability to catalyze the conversion of C2 and factor B to enzymatically active C2a and Bb and thereby prevents the formation of C4b2a and C3bBb, the amplification convertases of the complement cascade. Tissue specificity: Expressed on the plasma membranes of all cell types that are in intimate contact with plasma complement proteins. It is also found on the surfaces of epithelial cells lining extracellular compartments, and variants of the molecule are present in body fluids and in extracellular matrix. Similarity: Belongs to the receptors of complement activation (RCA) family. Contains 4 Sushi (CCP/SCR) domains. Domain: The first Sushi domain (SCR1) is not necessary for function. SCR2 and SCR4 provide the proper conformation for the active site on SCR3. PTM: The Ser/Thr-rich domain is heavily O-glycosylated.