Complement C4-B is one of the two isotypes of the complement component C4. Complement component C4 is a disulfide-bonded three-chain glycoprotein consisting of an α-chain, a β-chain, and a γ-chain. C4 plays an indispensable role in the activation of classical and lectin complement cascades. Upon activation, C4 is cleaved by C1s or MASP-2 to generate C4a and C4b fragments. C4b, the larger fragment, binds to the target surface via its reactive thioester and can be further processed to C4d and C4c, which can be used as biomarkers for complement activation. C4b also serves as a platform for the assembly of proteolytically active convertases that mediate downstream complement activation by cleavage of C3 and C5. C4B is known for its rapid and efficient formation of a covalent ester bond with hydroxyl group-containing substrates, while C4A forms a covalent amide bond with amino group-containing antigens more effectively. C4A may be functionally advantageous for ensuring the solubilization of antibody-antigen aggregates and clearance of immune complexes through binding to complement receptor CR1, while C4B functions more in the propagation of activation pathways leading to the formation of the membrane attack complex in attacking foreign antigens.