IC3b

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iC3b is a fragment of C3b, a protein in the vertebrate complement system, which is part of the immune system. It is produced when C3b is cut by the enzyme factor I. iC3b can be bound by receptors on white blood cells, helping to tag pathogens for attack (an opsonin). Unlike C3b, iC3b cannot bind factor B, so it does not amplify the complement response through the alternative pathway. Factor I can further cut iC3b to a fragment called C3d.