Opsonization

An opsonin (from the Greek opsōneîn, to prepare for eating) is any molecule that enhances phagocytosis by marking an antigen for an immune response or marking dead cells for recycling (i.e., causes the phagocyte to "relish" the marked cell).Opson in ancient Greece referred to the delicious side-dish of any meal, versus the sitos, or the staple of the meal.

Opsonization (also, opsonisation) is the molecular mechanism whereby molecules, microbes, or apoptotic cells are chemically modified to have stronger interactions with – to be more "delicious" to – cell surface receptors on phagocytes and NK cells. With the antigen coated in opsonins, binding to immune cells is greatly enhanced. Opsonization also mediates phagocytosis via signal cascades from cell surface receptors.

Opsonins aid the immune system in a number of ways. In a healthy individual, they mark dead and dying self cells for clearance by macrophages and neutrophils, activate complement proteins, and target cells for destruction through the action of natural killer (NK) cells.

All cell membranes have negative charges (Zeta potential) which makes it difficult for two cells to come close together. When opsonins bind to their targets they boost the kinetics of phagocytosis by favoring interaction between the opsonin and cell surface receptors on immune cells. This overrides the negative charges from cell membranes. This principle holds true for clearance of pathogens as well as dead or dying self cells.

Different opsonins perform different functions. Opsonin molecules include:

Antibodies are part of the adaptive immune response and are generated by B cells in response to antigen exposure. The Fab region of the antibody binds to the antigen, whereas the Fc region of the antibody binds to an Fc receptor on the phagocyte, facilitating phagocytosis. The antigen-antibody complex can also activate complement through the classical complement pathway. Phagocytic cells do not have an Fc receptor for immunoglobulin M (IgM), making IgM ineffective in assisting phagocytosis alone. However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin. IgG antibodies are also capable of binding immune effector cells via their Fc domain, triggering a release of lysis products from the bound immune effector cell (monocytes, neutrophils, eosinophils, and natural killer cells). This process, called antibody-dependent cellular cytotoxicity, can cause inflammation of surrounding tissues and damage to healthy cells.

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