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Urticaria
Urticaria (from the Latin ''urtica'', "nettle" from ''urere'', "to burn"),〔"urticaria": ''Oxford English Dictionary''. 2nd ed. 1989. OED Online. Oxford University Press. 2 May 2009.〕 commonly referred to as hives, is a kind of skin rash notable for pale red, raised, itchy bumps. Hives may cause a burning or stinging sensation.〔http://www.webmd.com/skin-problems-and-treatments/guide/hives-urticaria-angioedema〕 They are frequently caused by allergic reactions; however, there are many nonallergic causes. Most cases of hives lasting less than six weeks (acute urticaria) are the result of an allergic trigger. Chronic urticaria (hives lasting longer than six weeks) is rarely due to an allergy.
The majority of chronic hives cases have an unknown (idiopathic) cause. In perhaps as many as 30–40% of patients with chronic idiopathic urticaria, it is caused by an autoimmune reaction.
==Signs and symptoms==
Wheals (raised areas surrounded by a red base) from urticaria can appear anywhere on the surface of the skin. Whether the trigger is allergic or not, a complex release of inflammatory mediators, including histamine from cutaneous mast cells, results in fluid leakage from superficial blood vessels. Wheals may be pinpoint in size, or several inches in diameter.
Angioedema is a related condition (also from allergic and nonallergic causes), though fluid leakage is from much deeper blood vessels in the subcutaneous or submucosal layers. Individual hives that are painful, last more than 24 hours, or leave a bruise as they heal are more likely to be a more serious condition called urticarial vasculitis. Hives caused by stroking the skin (often linear in appearance) are due to a benign condition called dermatographic urticaria.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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