Complement Component 3b
null Complement component 3b is the large opsonic fragment generated when C3 convertases cleave C3, and it is the central hub of complement amplification. Its exposed internal thioester bond allows covalent attachment to nearby microbial and cellular surfaces, marking them for phagocytosis through complement receptors such as CR1. Surface-bound C3b recruits factor B to form the alternative-pathway C3 convertase C3bBb and, when incorporated into C4b2a, yields the C5 convertase that launches the terminal pathway. Factor H and factor I control C3b by cleaving it to iC3b. Recombinant C3b is used in opsonization, convertase, and regulator-binding assays.
