C3

Third component of complement, present in plasma at around 0.5-1mg ml-1. Both classical and alternate pathways converge at C3, which is cleaved to yield C3a, an anaphylotoxin, and C3b, which acts as an opsonin and is bound by CR1; C3b in turn can be proteolytically cleaved to iC3b (ligand for CR3) and C3dg by C3b-inactivator. C3b complexed with Factor B (to form C3bBb) will cleave C3 to give more C3b, although the C3bBb complex is unstable unless bound to properdin and a carbohydrate-rich surface. The C3b-C4b2a complex and C3bBb are both C5 convertases (cleave C5). Cobra venom factor is homologous with C3b but the complex of cobra venom factor, properdin and Factor Bb is insensitive to C3b-inactivator.