Human Complement C3 (C3) is synthesized as a single-chain pro-molecule (185 kDa) that then suffers several posttranslational modifications. Before being secreted as a mature protein, C3 is split into β-chain (645 residues and 70 kDa) and α chain (992 residues and 115 kDa) and forms a rare internal thioester bond. C3 plays a central role in the activation of all the three pathways of complement activation i.e. the classical, alternative, and lectin pathway. As C3 is the major complement component and participates in several stages of the immune response, its deficiency generally associated with higher susceptibility to severe bacterial infections and in some cases with autoimmune diseases such as systemic lupus erythematosus.