Complement C4a Unit Converter

Complement C4a is a small protein fragment released when complement protein C4 is cleaved during immune activation. It belongs to a group of molecules called anaphylatoxins, which are signaling proteins that trigger inflammatory responses as part of the innate immune system.

C4a is generated through two complement pathways: the classical pathway (activated by antibody-antigen complexes) and the lectin pathway (activated by sugar molecules on pathogens). It is the least potent of the three main anaphylatoxins (C3a, C4a, and C5a).

Measuring C4a detects active complement activation even when total C4 levels appear normal. This is because C4 is an acute-phase protein, so the same disease consuming it also stimulates the liver to produce more, masking activation through standard complement testing.

The parent protein C4 is synthesized primarily in the liver. C4a is generated when C4 is cleaved by the enzyme C1s (classical pathway) or MASP-2 (lectin pathway) during complement activation.