Complement regulatory protein CD46 is also known as membrane Cofactor Protein (MCP), is a type I membrane protein and is a regulatory part of the complement system. CD46 is expressed by all cells except erythrocytes. MCP 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, and also acts as a costimulatory factor for T-cells which induces the differentiation of CD4+ into T-regulatory 1 cells. In T-cells by binding to CD46, A number of viral and bacterial pathogens seem to exploit this property and directly induce an immunosuppressive phenotype. Defects in CD46 are a cause of susceptibility to hemolytic uremic syndrome atypical type 2 (AHUS2).