Allergy Hives

Urticaria (from the Latin urtica, nettle (whence It. ortica, Sp. ortiga, Pg. urtiga) urere, to burn"urticaria" The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 2 May 2009) (or hives) are a kind of skin rash notable for dark red, raised, itchy bumps. Hives are frequently caused by allergic reactions; however, there are many non-allergic causes. For example, 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) are rarely due to an allergy. The majority of patients with chronic hives have an unknown (idiopathic) cause. Perhaps as many as 30-40% of patients with chronic idiopathic urticaria will, in fact, have an autoimmune cause. Acute viral infection is another common cause of acute urticaria (viral exanthem). Less common causes of hives include friction, pressure, temperature extremes, exercise, and sunlight.

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 non-allergic, there is a complex release of inflammatory mediators, including histamine from cutaneous mast cells, resulting 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 non-allergic causes), though fluid leakage is from much deeper blood vessels. Individual hives that are painful, last >24 hours, or leave a bruise as they heal are more likely to be a more serious condition called urticaria pigmentosa. Hives caused by stroking the skin (often linear in appearance) are due to a benign condition called dermatographism.