Dictionary.net

Smaller fragment formed when C3 convertase splits C3 into C3a and C3b. C3a is a 77-amino acid peptide that includes a carboxy-terminal arginine which is crucial for its biological activities. C3a causes symptoms of immediate hypersensitivity (anaphylaxis) including smooth muscle contraction, mast cell histamine release, and local inflammation. It is considered an anaphylatoxin along with C4a, C5a, and C5a des-arginine.