iC3bfragmenttien

iC3b fragments are proteolytic products of C3b that arise during the regulation of the complement system. They form when C3b bound to a surface is cleaved by the serine protease factor I in the presence of cofactors that include factor H, CR1 (CD35), MCP (CD46), or C4b-binding protein. The resulting iC3b remains covalently attached to the surface and retains opsonizing activity while losing the ability to participate in further amplification of the cascade.

Historically, C3b functions as a key amplification fragment of complement. After C3b is deposited on a target, factor I-mediated cleavage with the appropriate cofactors converts C3b to iC3b, which cannot form new C3 convertases. This inactivation helps regulate inflammation and prevents unchecked complement activation. iC3b itself serves as an important bridge between innate and adaptive immunity by marking targets for phagocytosis without promoting further enzymatic cascade.

Receptors and function: iC3b is recognized by phagocyte receptors such as CR3 (CD11b/CD18) and CR4 (CD11c/CD18), which promotes uptake and digestion of opsonized particles, including pathogens, apoptotic cells, and immune complexes. Through its further processing, iC3b can be degraded to fragments like C3d, which can engage CR2 (CD21) on B cells and influence humoral immune responses.

Clinical and research relevance: The presence and pattern of iC3b deposition reflect local complement activity and regulation. Abnormal processing or regulatory defects can affect clearance of immune complexes and dying cells and may contribute to inflammatory or autoimmune conditions. In research, iC3b serves as a tool to study receptor-mediated phagocytosis and the regulatory aspects of the complement system.