Glycoprotein complement factor C2 is key in the classical and lectin pathway. Its proteolytic subunit is part of as well the C3 as C5 convertase. In CP C2 is cleaved by C1s and in the LP by MASP2. C2 binds to membrane bound C4b. After cleavage C2b is released and C2a forms with C4b the C3 convertase (C4bC2a). After proteolysis of C3 into C3a and C3b, the latter fragment associates with the C4bC2a complex in order to form the C5 convertase. The formation of both convertases is crucial in complement activation and to initiate the terminal part of the cascade. Synthesis of C2 is predominantly in hepatocytes, macrophages and lung epithelial cells. Individuals with C2-deficiency appear healthy, though there are some associations with SLE, atherosclerosis and increased risk to some type of infections.