opsoninos

Opsoninos are molecules that bind to pathogens or particulate material and mark them for attack by phagocytes, thereby enhancing phagocytosis. The term derives from the Greek opsōn, meaning “to prepare for eating.” In humans the most important opsonins are antibodies of the IgG class and fragments of the complement system, especially C3b and iC3b.

Antibodies bind to surface antigens on microbes via their Fab regions, while their Fc regions are recognized by Fc receptors on phagocytes (for example, FcγR receptors on macrophages and neutrophils). This Fc–Fc receptor interaction promotes engulfment. Complement-derived opsonins, such as C3b and iC3b, attach to microbial surfaces and engage complement receptors (notably CR1/CD35 and CR3/CD11b/CD18) on phagocytes. IgM can efficiently activate the classical complement pathway, leading to C3b deposition. Other soluble opsonins, such as mannose-binding lectin and C-reactive protein, can also enhance opsonization by binding microbes and interacting with phagocyte receptors.

The result is increased recognition and uptake of pathogens by phagocytes, aiding clearance from the circulation and tissues. Opsonization also facilitates the removal of immune complexes and contributes to antibody- and complement-mediated inflammatory responses.

Clinical relevance includes increased susceptibility to infections when opsonization is impaired, such as in deficiencies of C3 or other complement components, or in individuals with poor production of specific opsonizing antibodies. Individuals without a functioning spleen or with low levels of certain antibody subclasses are particularly prone to infections with encapsulated bacteria (for example Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis) due to reduced opsonization. Vaccination strategies often aim to elicit opsonizing antibodies to improve microbial clearance.