Cohen, J. John Department of Immunology, University of Colorado Medical School, Aurora, Colorado.
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A molecule that coats particles so that phagocytic cells can bind them more efficiently and then ingest them. The word opsonin is derived from the Latin opsonare, meaning to purchase or prepare food for the table. Phagocytes (“eating cells”) are specialized for trapping, binding to, ingesting, and digesting particles smaller than themselves. The most important of such particles are single-cell pathogens: bacteria, yeast, and some parasites. In humans, the major phagocytes are polymorphonuclear neutrophils (PMNs) and monocytes/macrophages, that is, the white cells of the blood and tissues. Pathogens have evolved techniques to evade phagocytosis, including motility (some bacteria can outswim a pursuing PMN), surface charge (which repels the phagocyte), and capsule formation (a frustrating slime layer). In an evolutionary countermove, most multicellular organisms have acquired various opsonins (see illustration). The simplest are lectins, which are proteins that bind certain saccharides (complex sugars) found on the surface of pathogens, but not on cells of the organism itself. The other end of the lectin (that is, the end opposite to that which binds the pathogen saccharide) is recognized by a receptor on the phagocyte. Thus, the opsonin forms a molecular bridge between phagocyte and pathogen, or, put another way, the opsonin covers the pathogen with molecular handles for the phagocyte to grab. Most creatures, from simple invertebrates to humans, have lectin opsonins; in humans, one of the best known is mannose-binding lectin. See also: Lectins; Macrophages; Pathogen; Phagocytosis
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