Complement C3 (C3) is the most abundant component of the complement system, with levels in human plasma of ~1.2-1.5 mg/ml. It is central to the activation of both the classical and alternative complement pathways, both of which lead to the proteolytic processing of C3 to C3a and C3b. C3 exists as a heterodimer with molecular masses of 115 kDa and 75 kDa, although it is readily processed to create C3a, C3b or C3c depending on the processed state. High C3 levels are seen in highly acute or chronic inflammation, hepatic cholestasis and during the third trimester of pregnancy.
Unwanted complement activation is a major cause of tissue damage in various pathological conditions and contributes to immune complex diseases.