opsonizing

Opsonization is the process by which foreign particles, most notably bacteria, are marked for ingestion and destruction by phagocytes. The marking is accomplished by opsonins, a group that includes antibodies of the IgG class and fragments of the complement system, especially C3b. Opsonization enhances the efficiency of phagocytosis and links innate and adaptive immune responses.

Mechanistically, opsonins coat the surface of a pathogen, creating ligands for receptors on phagocytes. Antibody-opsonized particles are recognized by Fc gamma receptors, while complement-tagged particles bind to complement receptors such as CR1 (CD35). This engagement promotes engulfment of the pathogen into a phagosome, followed by fusion with lysosomes and intracellular killing.

Opsonins arise from different pathways. The classical pathway involves antibodies, the lectin pathway involves mannose-binding lectin and other lectins, and the alternative pathway provides amplification. Deposited C3 fragments such as C3b and iC3b serve as key complement opsonins, while Fc regions of IgG provide antibody-mediated opsonization. Additional innate opsonins, including collectins like MBL and surfactant proteins in the lungs, can also promote opsonization.

Clinical relevance and biology. Effective opsonization is important for clearance of encapsulated bacteria and for the success of many vaccines, which induce protective opsonizing antibodies. Deficiencies in antibodies, complement components, or phagocytic receptors can lead to increased susceptibility to infections. Laboratory assays, such as opsonophagocytic killing tests, measure functional opsonizing activity. Pathogens may evade opsonization by masking surfaces or recruiting host regulators of complement.