Complement component 4
![Two pathways of the complement cascade.[4] Components and enzymes of the classical and alternative pathways of the complement cascade, which provides a complementary means for human and other systems to defend against foreign pathogens (see text). Not shown are elements of the lectin pathway.[5] Note, while the appended letters in this figure are lower cases, they are synonymous with the same designations appearing in upper case in the text.](/Images/godic/202501/05/Complement_pathway0806.png")
Complement component 4 (C4) is a protein involved in the complement system in human, primate, and other animals. It serves a number of critical functions in the complement system, including contributing component C4B, which bears a reactive thionolactone ring which covalently modifies the cell surfaces of infecting pathogens, and which is a part of the C3 convertase which readies the C3 protein for its role in the complement system. C4 defines the Chido/Rodgers blood group system, and it, and the gene encoding it, are being investigated for its possible role in schizophrenia risk and development.