This entry represents complement component C4-A. C4-A is proteolytically processed to generate subcomponents C4a and C4b by complement component activated C1s (classical pathway) or mannan-binding lectin-associated serine peptidases (lectin pathway). C4a is an anaphylatoxin, whereas C4b forms a complex with complement subcomponent C2a to form the C3/C5 convertase, which is stabilized by Mg2+. The C3/C5 convertase has trypsin-like specificity.