Opsonine

Opsonine, often used interchangeably with the term opsonin, denotes any molecule that binds to a microbe or particle and targets it for attack by phagocytes. In the immune system, opsonization enhances the recognition and uptake of pathogens by cells such as macrophages and neutrophils, increasing the efficiency of clearance.

The principal opsonins are antibodies—especially immunoglobulin G (IgG) and, to a lesser extent, IgM—and complement fragments such as C3b and C3d. Additional opsonins include collectins like mannose-binding lectin and certain acute-phase proteins such as C-reactive protein. Opsonins act by binding to microbial surfaces and presenting molecular handles that are recognized by phagocyte receptors, including Fc gamma receptors and complement receptors (e.g., CR1, CR3). This bridging promotes phagocytosis and activates intracellular killing mechanisms.

Opsonization is important for defense against extracellular bacteria, fungi, and other microbes. Defects in opsonization—due to antibody deficiencies, low complement activity, or receptor mutations—can lead to recurrent or severe infections, particularly with encapsulated bacteria. Conversely, some pathogens have evolved mechanisms to resist opsonization.

In clinical and research settings, enhancing opsonization is a strategy for vaccines and antibody therapies. Therapeutic monoclonal antibodies can function as opsonins to promote clearance of pathogens or malignant cells, while vaccine design often aims to elicit high-titer, opsonizing antibodies. Measurements of opsonizing activity and receptor engagement are used to evaluate immune function and therapeutic efficacy.

See also: Opsonization, humoral immunity, complement system, phagocytosis.