CD55, also designated decay accelerating factor (DAF), is a 60-70 kDa glycosyl-phosphatidylinositol (GPI) anchored protein. CD55 is a member of the family of proteins that protect host tissue from damage by autologous complement. CD55 was first recognized as a species restricting factor operating at the level of C3 activation. It binds C3b and C4b to inhibit formation and half-life of the C3 convertases. CD55 expression increases upon T cell activation and in the presence of phorbol esters antibodies to CD55 (e.g. monoclonal antibody D17) are mitogenic. CD55 is broadly distributed among cells in contact with serum, including both haematopoietic and non-haematopoietic cells. Although CD55 does not have an essential role in controlling hemolysis of erythrocytes, it has an important role in regulation of the deposition of C3 on nucleated cells.