opsonization

Definition: The process of coating a particle, such as a microbe, to target it for phagocytosis is called opsonization, and substances that do this are opsonins.

The efficiency of phagocytosis is greatly enhanced when microbes are opsonized by specific proteins (opsonins) for which the phagocytes express high-affinity receptors. The major opsonins are IgG antibodies, the C3b breakdown product of complement, and certain plasma lectins, notably MBL, all of which are recognized by specific receptors on leukocytes.

Receptors for opsonins promote phagocytosis of microbes coated with various proteins and deliver signals that activate the phagocytes.

Opsonins include antibodies, complement proteins, and lectins.

One of the most efficient systems for opsonizing particles is coating the particles with IgG antibodies, which are termed specific opsonins and are recognized by the high-affinity Fcγ receptor of phagocytes, called FcγRI.

Components of the complement system, especially fragments of the complement protein C3, are also potent opsonins, because these fragments bind to microbes and phagocytes express a receptor, called the type 1 complement receptor (CR1), that recognizes breakdown products of C3 (discussed later).

These complement fragments are produced when complement is activated by either the classical (antibody-dependent) or the alternative (antibody-independent) pathway. Many bacteria can activate the alternative pathway and produce complement proteins that efficiently opsonize the bacteria in the absence of antibody molecules.

A number of plasma proteins, including mannose-binding lectin (MBL), fibronectin, fibrinogen, and C-reactive protein, can also coat microbes and are recognized by receptors on phagocytes. For example, a macrophage cell surface receptor called the C1q receptor binds microbes opsonized with plasma MBL, and integrins bind fibrinogen-coated particles.