Acts as a cofactor for complement factor I, a serine protease which protects autologous cells against complement-mediated injury by cleaving C3b and C4b deposited on host tissue. Also acts as a decay-accelerating factor, preventing the formation of C4b2a and C3bBb, the amplification convertases of the complement cascade. Seems to act as a costimulatory factor for T-cells. May play a crucial role in early embryonic development by maintaining fetomaternal tolerance. Interacts with C3b. Belongs to the receptors of complement activation (RCA) family.