Human Complement C3 (C3) is synthesized as a single-chain pro-molecule (185 kDa) that tsuffers several posttranslational modifications. Before being a mature protein, C3 is split into beta-chain (70 kDa) and alpha chain (115 kDa) and forms a rare internal thioester bond. C3 plays a central role in the activation of the complement system. It’s processing by C3 convertase is the central reaction in both classical and alternative complement pathways. After activation C3b can bind covalently, via its reactive thioester, to cell surface carbohydrates or immune aggregates.