iC3b is a processed fragment of C3b formed through cleavage by complement factor I with a suitable partner such as complement factor H. It functions as an opsonin: deposition on a target supports recognition and uptake by phagocytes, including through complement receptor 3. [ref]
Processing does not simply erase every biological effect of the original protein. It changes the interactions available to the resulting fragment. In a 1983 experiment, cultured monocytes on appropriately coated surfaces showed complement-associated phagocytosis without the appreciable hydrogen peroxide release seen with the IgG-coated comparison.[ref]
The comparison separates target uptake from an oxidative response. Measuring either alone would miss that separation; receptor engagement, ingestion and production of reactive substances are distinct experimental readouts. The older paper writes the fragment as C3bi; iC3b is the corresponding modern notation.

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