CD55 is a member of the RCA (regulators of complement activation) family. RCA proteins is characterized by the presence of four to 30 SCRs (short consensus repeats also called CCPs for control protein modules) in their plasmaexposed regions. CD55 containing four SCR modules is involved in the regulation of the complement cascade. CD55 is known to bind CD97 via the first SCR. It also binds physiologically generated 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. It is known that viruses and bacteria also utilize multiple SCR sites for infection.