Opsonin

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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.

1 Mechanism
2 Examples
3 See also
4 References
5 External links

[edit] Mechanism

Both the membrane of a phagocytising 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. 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.

[edit] Examples

Examples of opsonin molecules include the IgG and IgA antibodies and the C3b, C4b, and iC3b components of the complement system. The most important are C3b and IgG.^[1]