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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. 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.

Opsonins are freely circulating serum molecules which are produced to attach to the surface of microbes, rendering them more attractive to phagocytes.

Examples of opsonins include IgG antibody and the C3b molecule of the complement system. Each has receptors for both foreign particle and host phagocyte.

Opsonisation can itself stimulate the local activation of complement, further enhancing the local production of C3b opsonin and phagocytosis.

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.[1]

The Opsonic Index

The Opsonic Index

Is calculated by dividing the number of bacteria in blood per phagocyte when an immune response has been mounted compared to that previously. Each figure is an average for identical unit volumes or smear counts.

Veratrum viride will raise the opsonic index against the Diploccus Pneumonia, 70 to 109 per cent. (Boericke)