C3a (complement)

C3a is a one of the proteins formed by the cleavage of complement component 3; the other is C3b. It stimulates mast cell degranulation, thus triggering an immune response.

C3a plays an important role in chemotaxis, though not as important a role as C5a.

It is also an anaphylatoxin and the precursor of the important cytokine (adipokine) ASP through its interaction with carboxypeptidase B.