[2] Complement components C3, C4 and C5 are large glycoproteins that have important functions in the immune response and host defense.

[4] A-fragments form distinct structural domains of approximately 76 amino acids, coded for by a single exon within the complement protein gene.

The C3a, C4a and C5a components are referred to as anaphylatoxins:[4][5] they cause smooth muscle contraction, vasodilation, histamine release from mast cells, and enhanced vascular permeability.

[5] They also mediate chemotaxis, inflammation, and generation of cytotoxic oxygen radicals.

[5] The proteins are highly hydrophilic, with a mainly alpha-helical structure held together by 3 disulfide bridges.