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. Plays a crucial role in early embryonic development by maintaining fetomaternal tolerance. Also acts as a costimulatory factor for T-cells which favors IL-4 secretion. Interacts with C3b. Ubiquitously expressed (at protein level). Highly expressed in trophoblasts at 7.5 dpc, and in the maternally derived decidual tissues until 16 dpc. Expressed only at low levels in the embryo itself. Death between 9.5 and 13.5 dpc from developmental arrest. Belongs to the receptors of complement activation (RCA) family.