An opsonin is any molecule that acts as a binding enhancer for the process of phagocytosis. 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 can not 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.