opsoninsmolecules

Opsonins are molecules that tag pathogens and other particles to enhance their recognition and uptake by phagocytes, a process known as opsonization. The best characterized opsonins are antibodies of the IgG and IgM isotypes and complement fragments such as C3b and iC3b. Other soluble proteins, including lectins such as mannose-binding lectin and acute-phase proteins like C-reactive protein, can also function as opsonins in certain contexts. By coating a microbe, opsonins convert a non-specific encounter into a more efficient interaction with phagocytic receptors.

Phagocytes such as neutrophils and macrophages have receptors that recognize opsonins: Fc receptors for antibodies, notably Fcγ receptors for IgG, and complement receptors such as CR1 (CD35) for C3b/C4b and CR3 (CD11b/CD18) for iC3b. Binding of opsonized particles promotes attachment and engulfment, leading to phagosome formation, fusion with lysosomes to form phagolysosomes, and destruction of the ingested material through reactive oxygen species and hydrolases.

Types and relevance: IgG- and IgM-mediated opsonization is central to defense against many bacteria, especially encapsulated species like Streptococcus pneumoniae and Haemophilus influenzae. C3b-mediated opsonization provides a broad mechanism to enhance clearance of bacteria, immune complexes, and damaged cells. Deficiencies in antibodies or complement components can impair opsonization and increase susceptibility to infection; some pathogens also evade opsonization by presenting capsules or other surfaces that limit antibody or complement deposition.

Clinical notes: Therapeutically, intravenous immunoglobulin (IVIG) supplies broad opsonizing antibodies, and monoclonal antibodies used in infectious disease and cancer can enhance opsonization of target pathogens or cells. Inflammation and acute-phase responses can modulate the abundance and activity of opsonins, adjusting the efficiency of opsonization according to context.