CD55 is also known as DAF or decay­accelerating factor, is a member of the RCA (regulators of complement activation) family characterized by four to 30 SCRs (short consensus repeats) in their plasma-exposed regions. CD55 /DAF is a 70 kDa membrane protein that regulates the complement system on the cell surface. It prevents the assembly of the C3bBb complex (the C3-convertase of the alternative pathway) or accelerates the disassembly of preformed convertase, thus blocking the formation of the membrane attack complex. This glycoprotein is broadly distributed among hematopoietic and non-hematopoietic cells. It is a determinant for the Cromer blood group system. CD55 is known to bind CD97 via the first SCR. It also binds physiologically­generated C3 convertases with its second and third SCRs. Binding results in an accelerated “decay”, or dissociation of active C3 convertases, thus blocking the development of C’ attack complexes on nonforeign cells. Finally, viruses and bacteria are also known to utilize multiple SCR sites for infection.