Antibody opsonization

Antibody opsonization is the process by which a pathogen is marked for ingestion and eliminated by a phagocyte.

Opson in ancient Greece referred to the savory side-dish of a meal.

Under normal inflammatory circumstances, microbial pathogen-associated molecular patterns (PAMPs) can bind to the endocytic pattern recognition receptors (PRRs) of phagocytes, which mediates neutrophil or macrophage phagocytosis. As well as endocytic PRRs, phagocytes also express opsonin receptors such as Fc receptor and complement receptor 1 (CR1). Should the microbe be coated with opsonising antibodies or C3b complement, the co-stimulation of endocytic PRR and opsonin receptor increases the efficacy of the phagocytic process, enhancing the lysosomal elimination of the infective agent. This mechanism of antibody-mediated increase in phagocytic efficacy is named opsonization.

Opsonization involves the binding of an opsonin, e.g., antibody, to an epitope on an antigen. After opsonin binds to the membrane, phagocytes are attracted to the pathogen. The Fab portion of the antibody binds to the antigen, whereas the Fc portion of the antibody binds to an Fc receptor on the phagocyte, facilitating phagocytosis. The receptor-opsonin complex can also create byproducts like C3b and C4b which are important components of the complement system. These components are deposited on the cell surface of the pathogen and aid in its destruction.

The cell can also be destroyed by a process called antibody-dependent cell-mediated cytotoxicity, in which the pathogen does not need to be phagocytosed to be destroyed. During this process, the pathogen is opsonized and bound with the antibody IgG via its Fab domain. Then the antibody binds an immune effector cell via its Fc domain and this binding triggers a release of lysis products from the bound immune effector cell (monocytes, neutrophils, eosinophils and natural killer cells). This process can cause inflammation of surrounding tissues and damage to healthy cells.

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