Complement Component C3a is an anaphylotoxin polypeptide that is proteolytically generated from C3. C3a formation is common to all three pathways of complement activation: classical (antibody-mediated), lectin and alternative. It binds C3a R on myeloid peripheral blood leukocytes, activated lymphocytes, endothelial and internal organ epithelial cells. It activates mast cells, neutrophils, and macrophages, enhances Th2 inflammatory reactions, promotes Th1 cell maturation, and inhibits Treg differentiation. It also stimulates leukocyte chemotaxis and smooth muscle contraction. Removal of the C-terminal arginine generates C3adesArg/ASP that binds the C5L2 receptor (GPR77) and stimulates triglyceride synthesis in adipocytes.