What is C3d?
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An inactivated fragment of complement 3b (C3b). Factor H makes C3b susceptible to factor I (formerly called KAF, C3bINF, or enzyme 3b inactivator) to form iC3b. Then factor I and a trypsin-like proteolytic enzyme further cleave iC3b into C3c and C3dg. Serum proteases degrade C3dg into complement 3d (C3d) and C3g.