Complement component C4 is a non-enzymatic component of C3 and C5 convertases and essential for the propagation of the classical complement pathway. It covalently binds to immunoglobulins and immune complexes and enhances the solubilization of immune aggregates and clearance through CR1 on erythrocytes. C4 is cleaved into the fragments C4a and C4b by MASP-2 protease. C4A isotype is responsible for effective binding to form amide bonds with immune aggregates or antigens, whereas C4B isotype catalyses the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. This entry represents the macroglobuline 1 (MG1) found at the N-terminal of C4 [1].

1: Structural basis for activation of the complement system by component C4 cleavage. Kidmose RT, Laursen NS, Dobo J, Kjaer TR, Sirotkina S, Yatime L, Sottrup-Jensen L, Thiel S, Gal P, Andersen GR; Proc Natl Acad Sci U S A. 2012;109:15425-15430. PMID:22949645