opsonins

Opsonins are soluble host molecules that tag pathogens, dead cells, and debris for uptake by phagocytes. By binding to microbial surfaces and presenting ligands to phagocyte receptors, they convert invaders into targets for efficient engulfment and destruction.

The most important opsonins are antibodies (primarily the IgG class) and complement fragments such as C3b and C3d. Other serum and mucosal proteins can also act as opsonins, including mannose-binding lectin (MBL), ficolins, surfactant proteins A and D, and C-reactive protein. These molecules bind to microbes and create a bridge to phagocytes, enhancing recognition and clearance.

Phagocytes recognize opsonins through specific receptors. Fc receptors on neutrophils and macrophages bind IgG-coated particles, while complement receptors (such as CR1 and CR3) bind C3b or C3d. Opsonization can occur via different cascades: antibody-mediated opsonization is often accompanied by activation of the classical complement pathway through C1q; lectin pathway activation by MBL also promotes C3b deposition; the alternative pathway can generate C3b independently of antibodies. The result is a higher likelihood of phagocytosis and more efficient microbial killing.

Clinical relevance includes the crucial role of opsonization in defending against encapsulated bacteria that resist non-opsonic phagocytosis. Deficiencies in antibodies or in complement components that generate C3b reduce opsonization and raise infection risk. Therapeutic approaches such as intravenous immunoglobulin or targeted complement support can help restore opsonizing activity in affected individuals.