Opsonin

An opsonin is any molecule that targets an antigen for an immune response. However, the term is usually used in reference to molecules that act as a binding enhancer for the process of phagocytosis, especially antibodies, which coat the negatively-charged molecules on the membrane. Molecules that activate the complement system are also considered opsonins. Phagocytic cells do not have an FC receptor for IgM, making IgM ineffective in assisting phagocytosis. However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin. Opsonins can also refer to molecules that target a cell for destruction through the action of Natural Killer cells. Only opsonins that enhance phagocytic activity will be further described in this article.

Mechanism
Both the membrane of a phagocytosing cell and its target have a negative charge (zeta-potential), making it difficult for the two cells to come close together. Once the opsonins attach to the target, the negative charged is masked. Take note that the negative charge of the target doesn't disappear. The opsonin simply overrides the charge, making it easier for white blood cells (phagocytic cells), to undergo phagocytosis. During the process of opsonization (also, opsonisation), antigens are bound by antibody or complement molecules. Phagocytic cells express receptors, CR1 and Fc receptors, that bind opsonin molecules, C3b and antibody, respectively. With the antigen coated in these molecules, binding of the antigen to the phagocyte is greatly enhanced. In fact, 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 IgM
 * Components of the complement system: C3b, C4b, and iC3b
 * Mannose-binding lectin (initiates the formation of C3b)

The most important are IgG and C3b.