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.
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
are bound by antibody
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 of opsonin molecules include:
The most important are IgG and C3b.