Complement component 3

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C3 is the most abundant complement component. It is a component of the alternative complement pathway, and is synthesised mainly in the liver. Cleavage of C3 into C3a and C3b is central to complement activation and can occur by the:

The cleaved C3b molecule exposes several binding sites, including a thioester one which is relocated to a position adjacent to the first macroglobulin domain. C3b attaches to target surfaces such as pathogens and binds to factor B. Factor B is then cleaved. This produces a C3bBb complex which has C3 convertase activity. More C3 is then cleaved, amplifying the response and leading to C3b2 complexes with C5 convertase activity.

Indirect implications