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. Also acts 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 (By similarity). Interacts with C3b. Interacts with C4b. Interacts with moesin/MSN. Inner acrosomal membrane of spermatozoa. Present in blood and sperm. Isoform 2, but not isoform 1, is present at the erythrocyte membrane (at protein level). Sushi domains 3 and 4 are the most important for interaction with C3b and C4b. N-glycosylated. Probably less N-glycosylated in testis.