Opsonin
any molecule that identifies and marks cells or substances for destruction by the immune system
Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. eaten) by phagocytes (cells that specialise in phagocytosis, i.e. cellular eating). Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens (such as bacteria), cancer cells, aged cells, dead or dying cells (such as apoptotic cells), excess synapses, or protein aggregates (such as amyloid plaques). Opsonins help clear pathogens, as well as dead, dying and diseased cells.
Opsonins were discovered and named "opsonins" in 1904 by Almroth Wright and Stewart Ranken Douglas, who found that incubating bacteria with blood plasma enabled phagocytes to phagocytose (and thereby destroy) the bacteria. They concluded that: "We have here conclusive proof that the blood fluids modify the bacteria in a manner which renders them a ready prey to the phagocytes. We may speak of this as an "opsonic" effect (opsono - I cater for; I prepare victuals for), and we may employ the term "opsonins" to designate the elements in the blood fluids which produce this effect."
Subsequent research found two main types of opsonin in blood that opsonised bacteria: complement proteins and antibodies. However, there are now known to be at least 50 proteins that act as opsonins for pathogens or other targets.
The public source identifies “Opsonin” as any molecule that identifies and marks cells or substances for destruction by the immune system. This brief keeps that definition visible, then builds a research path around Opsonin, molecule and identifies.
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This entry incorporates text from “Opsonin” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.