Opsonin

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.