iC3b is a proteolytically inactive product of the complement cleavage fragment C3b[1] that still opsonizes microbes, but cannot associate with factor B, thus preventing amplification of the complement cascade or activation through the alternative pathway.
References
^Robbins Basic Pathology 8th ed 2007. R Cotran, S Robbins, V Kumar, J Perkins. W.B. Saunders Company.