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. (Microbial infection) A number of viral and bacterial pathogens seem to bind MCP in order to exploit its immune regulation property and directly induce an immunosuppressive phenotype in T-cells. Acts as a receptor for adenovirus subgroup B2 and Ad3, cultured measles virus and herpesvirus 6(PubMed:10972291, PubMed:12663806, PubMed:12724329, PubMed:12915534, PubMed:14566335, PubMed:15047806, PubMed:15078926, PubMed:15919905, PubMed:16254377). May act as a receptor for pathogenic bacteria Neisseria and Streptococcus pyogenes(PubMed:7708671, PubMed:9379894, PubMed:11260136, PubMed:11971006).