Complement C4 is a key component of the immune system involved in defense against infection. It is activated through the classical and lectin pathways of the complement system, playing a role in the formation of the membrane attack complex (MAC) upon pathogen attack. Once activated, C4 helps opsonize pathogens and participates in the inflammatory process. Autoimmune diseases can be associated with abnormalities in the complement system, including C4, and testing its levels can be helpful in diagnosis.