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Urushiol is an oily organic allergen found in plants of the family Anacardiaceae, especially ''Toxicodendron'' ''spp.'' (e.g., poison oak, Lacquer Tree, poison ivy, poison sumac) and also in mangos. In sensitive individuals, urushiol can cause an allergic skin rash on contact,〔Buck Tilton (2004) ''Wilderness First Responder: How to Recognize, Treat, and Prevent Emergencies in the Backcountry'', Globe Pequot, 320 pages ISBN 0-7627-2801-9〕 known as urushiol-induced contact dermatitis. The name urushiol is derived from the Japanese word for the lacquer tree, .〔Oxford English Dictionary〕 The oxidation and polymerization of urushiol in the tree's sap in the presence of moisture allows it to form a hard lacquer, which is used to produce traditional Chinese, Korean and Japanese lacquerware. == Characteristics == Urushiol is a pale-yellow liquid with a specific gravity of 0.968 and a boiling point of . It is soluble in alcohol, ether, and benzene.〔''Hawley's Condensed Chemical Dictionary'', 14th Edition, 2002, John Wiley & Sons, Inc. ()〕 Urushiol is a mixture of several closely related organic compounds. Each consists of a catechol substituted with an alkyl chain that has 15 or 17 carbon atoms. The alkyl group may be saturated or unsaturated. The exact composition of the mixture varies, depending on the plant source. Whereas western poison oak urushiol contains chiefly catechols with C17 side-chains,〔C. Michael Hogan (2008) ''Western poison-oak: Toxicodendron diversilobum'', GlobalTwitcher, ed. Nicklas Stromberg ()〕 poison ivy and poison sumac contain mostly catechols with C15 side-chains. The likelihood and severity of allergic reaction to urushiol is dependent on the degree of unsaturation of the alkyl chain. Less than half of the general population experience a reaction with the saturated urushiol alone, but over 90% do so with urushiol that contains at least two degrees of unsaturation (double bonds). Longer side chains tend to produce a stronger reaction.〔McGovern Tomas, Barkley Theodore (1998) "Review Botanical Dermatology", International Journal of Dermatology, vol 37, pp 321-334〕 This suggests that the allergic contact dermatitis is caused by Van der Waals forces.〔 Before the urushiol has been absorbed by the skin, it can be removed with soap and water. However, time is of the essence, as 50% of the urushiol can be absorbed within 10 minutes. Once urushiol has penetrated into the skin, attempting to remove it with water is ineffective.〔 Once absorbed by the skin it is recognized by the immune system's dendritic cells, otherwise called Langerhans cells. These cells then migrate to the lymph nodes, where they present the urushiol to T-lymphocytes and thus recruit them to the skin. Once in the skin, the T-lymphocytes cause pathology through the production of cytokines and cytotoxic damage to the skin.〔Gober D. Michael et al. (2008) "Human Natural Killer T Cells infiltrate into the Skin at Elicitation Sites of Allergic Contact Dermatitis" Journal of investigative Dermatology, vol 128, pp1460-1469〕 Urushiol is an oleoresin contained within the sap of poison ivy and related plants, and after injury to the plant, or late in the fall, the sap leaks to the surface of the plant, where under certain temperature and humidity conditions the urushiol becomes a blackish lacquer after being in contact with oxygen.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Urushiol」の詳細全文を読む スポンサード リンク
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