opsonized

Opsonized refers to a particle, typically a microbe, that has been marked by opsonins to promote its recognition and ingestion by phagocytes. Opsonins are molecules such as antibodies—most notably IgG—and components of the complement system, particularly C3b and its fragments (e.g., iC3b). When a pathogen is coated with these molecules, phagocytes display receptors that bind opsonins, increasing the efficiency of phagocytosis.

In humans the main phagocytes are neutrophils and macrophages. The Fc gamma receptors (FcγR) bind the Fc region of IgG, while complement receptors (such as CR1 and CR3) recognize C3b and iC3b. The binding triggers engulfment, opens the respiratory burst, and leads to microbial killing. Opsonization can occur via the classical, alternative, or lectin complement pathways, depending on the initiating trigger.

Encapsulated bacteria, including Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, are particularly dependent on opsonization for effective clearance; without sufficient opsonizing antibodies or complement, they may resist phagocytosis and cause invasive disease. Conversely, opsonization is not always required, because phagocytes can also ingest some targets through pattern-recognition receptors in an opsonin-independent process, albeit more slowly.

Deficiencies in antibodies or complement components that impair opsonization increase susceptibility to infections. Vaccines and immunoglobulin therapies can augment opsonization by providing or boosting opsonizing antibodies.