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Action of opsonins; a phagocytic cell recognises the opsonin on the surface of an antigen

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 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 (alternatively opsonisation), antigens are bound by antibody and/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. 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.[1]

See also

The example of opsonine is the lectin pathway. It is a type of the complement system. Mannan-binding lectin proteins play a major role in activation of this pathway; belongs to the same family as C1q, a family called the collectins.


  1. ^ Immunology at MCG 1/phagocyt

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