C3bcoated

C3b-coated refers to surfaces or particles onto which the complement component C3b has been covalently deposited during activation of the complement system. C3b is generated when C3 is cleaved by C3 convertases from the classical or lectin pathways (C4b2a) or the alternative pathway (C3bBb). C3b forms a covalent thioester bond with surface molecules, tagging the surface for recognition by immune cells. As an opsonin, C3b promotes phagocytosis by cells bearing complement receptors such as CR1 (CD35) and CR3 (CD11b/CD18), and by erythrocytes via CR1 that ferry immune complexes to the liver and spleen for clearance. C3b coating can also participate in the assembly of C5 convertases, enabling formation of C5b-9 on the surface if regulation fails.

Regulation prevents collateral damage: host surfaces express factor H and factor I that inactivate C3b to iC3b and C3d, limiting self-tissue injury. Pathogens may evade by binding factor H or expressing inhibitors that disrupt C3b function, helping them resist opsonization.

Physiological and clinical relevance: C3b-coated surfaces are central to opsonization and clearance of pathogens and immune complexes; dysregulation can contribute to inflammatory or autoimmune processes. In research settings, C3b coating of particles or cells is used to study opsonophagocytosis and to assess complement activity, and it informs understanding of immune clearance mechanisms and related diseases.