Opsonin

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

An opsonin is any molecule that acts as a binding enhancer for the process of phagocytosis, for example, by coating the negatively-charged molecules on the membrane.

Mechanism

Both the membrane of a phagocytosing cell, as well as its target, have a negative charge (zeta-potential), making it difficult for the two cells to come close together. During the process of opsonization, antigens are bound by antibody and/or complement molecules. Phagocytic cells express receptors that bind opsonin molecules. These include the Fc receptors. With the antigen coated in these molecules, binding of the antigen to the phagocyte is greatly enhanced. Most phagocytic binding cannot occur without opsonization of the antigen.

Furthermore, opsonization of the antigen and subsequent binding to an activated phagocyte will cause increased expression of complement receptors on neighboring phagocytes.

Examples

Examples of opsonin molecules include:

• antibodies: IgG and IgA
• components of the complement system: C3b, C4b, and iC3b
• Surfactant
• Mannose-binding lectin (initiates the formation of C3b)

The most important are IgG and C3b.

See also

Antibody opsonization.

External links

• Opsonins at the US National Library of Medicine Medical Subject Headings (MeSH)