C3 is a type of complement, which is found in normal human serum and tissue fluids as an enzymatically active protein. It consists of about 30 soluble proteins as well as membrane-bound proteins, collectively referred to as the complement system. Under physiological conditions, most of the complement components in serum exist in the form of inactive enzyme precursors, only under the action of certain activators, or on specific surfaces, the complement components are activated at once, a chain of enzymatic reactions can occur, and can show a variety of biological activities, such as lysis, bactericidal, cytotoxicity, phagocytosis, immunoadhesion, neutralization and lysis of viral and inflammatory mediators and other roles. The activation of the complement system can be divided into: classical and bypass pathways, immune complexes, C-reactive protein, mitochondrial membranes, and activators such as mannose-binding lectin, which can sequentially activate C1, C4, C2, and C3 to form the C3 and C5 convertases. This activation pathway is called the classical pathway, also known as the traditional pathway. Without C1, C4, C2 activation, activators such as bacterial endotoxin come directly to activate C3, leading to the occurrence of successive enzymatic reactions with the participation of factors B and D. This is called the bypass pathway. The content of each component of complement varies greatly, with C3 being the most abundant, and is synthesized mainly by macrophages and the liver. Under the action of C3 converting enzyme, C3 is cleaved into C3A and C3B2 fragments, which play important roles in both the classical and bypass activation pathways of complement.