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. {ECO:0000269|PubMed:10843656, ECO:0000269|PubMed:12540904}. (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. (Microbial infection) Acts as a receptor for Adenovirus subgroup B2 and Ad3. {ECO:0000269|PubMed:12915534, ECO:0000269|PubMed:14566335, ECO:0000269|PubMed:15047806, ECO:0000269|PubMed:15078926, ECO:0000269|PubMed:15919905, ECO:0000269|PubMed:16254377}. (Microbial infection) Acts as a receptor for cultured Measles virus. {ECO:0000269|PubMed:10972291, ECO:0000269|PubMed:20010840, ECO:0000269|PubMed:7534417, ECO:0000269|PubMed:8371352, ECO:0000269|PubMed:8402913}. (Microbial infection) Acts as a receptor for Herpesvirus 6/HHV-6. . (Microbial infection) May act as a receptor for pathogenic bacteria Neisseria and Streptococcus pyogenes (PubMed:11260136, PubMed:11971006, PubMed:7708671, PubMed:9379894). (Microbial infection) Plays a minor role in fiber protein- mediated human adenovirus 55 infection; via its interaction with fiber protein (PubMed:32581096). Plays a minor role in fiber protein-mediated human adenovirus 14a infection (PubMed:32581096). . This is the function of Ensembl gene identifier ENSG00000117335 (CD46, CD46 molecule).