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. Both the membrane of a phagocytising cell, as well as its next meal, 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. 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
IgG antibody and the
C3b,
C4b, and
iC3b components of the
complement system.
Antibody opsonization.
