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. May be involved in the fusion of the spermatozoa with the oocyte during fertilization. May act as a costimulatory factor for T-cells which induces the differentiation of CD4+ into T-regulatory 1 cells. T-regulatory 1 cells suppress immune responses by secreting interleukin-10, and therefore are thought to prevent autoimmunity. Interacts with C3b. Interacts with C4b. Interacts with moesin/MSN. Inner acrosomal membrane of spermatozoa. Broadly expressed. Expressed on erythrocytes, leukocytes, granulocytes, platelets, vascular endothelial cells and kidney epithelial cells. Not or weakly expressed in muscle cells and skin (at protein level). N-glycosylated. May be O-glycosylated.