Precursor of non-enzymatic components of the classical, lectin and GZMK complement pathways, which consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system. {ECO:0000269|PubMed:12878586, ECO:0000269|PubMed:18204047, ECO:0000269|PubMed:22691502, ECO:0000269|PubMed:22949645, ECO:0000269|PubMed:2395880, ECO:0000269|PubMed:32769120, ECO:0000269|PubMed:35428691, ECO:0000269|PubMed:39814882, ECO:0000269|PubMed:39914456, ECO:0000269|PubMed:8538770}. [Complement C4b-A]: Non-enzymatic component of C3 and C5 convertases (PubMed:8538770). Generated following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway), it covalently attaches to the surface of pathogens, where it acts as an opsonin that marks the surface of antigens for removal (PubMed:27738201, PubMed:8538770). It then recruits the serine protease complement C2b to form the C3 and C5 convertases, which cleave and activate C3 and C5, respectively, the next components of the complement pathways (PubMed:12878586, PubMed:18204047, PubMed:2387864, PubMed:6906228). Complement C4b-A isotype is responsible for effective binding to form amide bonds with immune aggregates or protein antigens, while complement C4b-B isotype catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens (PubMed:8538770). {ECO:0000269|PubMed:12878586, ECO:0000269|PubMed:18204047, ECO:0000269|PubMed:2387864, ECO:0000269|PubMed:27738201, ECO:0000269|PubMed:6906228, ECO:0000269|PubMed:8538770}. [C4a anaphylatoxin]: Putative humoral mediator released following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway) (PubMed:6167582). While it is strongly similar to anaphylatoxins, its role is unclear (PubMed:25659340). Was reported to act as a mediator of local inflammatory process; however these effects were probably due to contamination with C3a and/C5a anaphylatoxins in biological assays (PubMed:25659340). {ECO:0000269|PubMed:6167582, ECO:0000303|PubMed:25659340}. This is the function of Ensembl gene identifier ENSG00000244731 (C4A, complement C4A (Chido/Rodgers blood group)).