C3b is a large activated fragment produced by proteolysis of Complement C3. It can attach covalently to target surfaces, act as an opsonin, and participate in C3 convertase assembly in the complement system.[ref]
Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others. Introduction and Results: C3b formation, surface attachment and regulatory interactions
Processing changes its functions. Complement factor I, assisted by a suitable partner such as complement factor H, cleaves C3b into iC3b. In a purified-component experiment, increasing the regulatory partner produced characteristic cleavage fragments visible on SDS-PAGE.[ref]
Think of two distinct experimental measurements: a surface assay detects deposited C3b, while an activity assay tests further complement system amplification. Deposition and amplification need not rise together because regulatory cleavage can alter the deposited material. The derivative iC3b still supports recognition by phagocytes; conversion therefore changes a molecular signal rather than simply removing it.[ref]
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