The smaller fragment generated from the cleavage of complement c3 by C3 CONVERTASE. C3a, a 77-amino acid peptide, is a mediator of local inflammatory process. It induces smooth muscle contraction, and histamine release from mast cells and leukocytes. C3a is considered an anaphylatoxin along with complement c4a; COMPLEMENT C5A; and complement c5a, des-arginine.