The oleoresin urushiol, which flows through the poison ivy plant and its relatives, causes the allergic reaction (a red, itchy rash). Urushiol is a sticky, clear oil containing catechols and other phenolic resins that act as a powerful hapten (a substance that does not stimulate antibody formation but reacts selectively in vitro with an antibody).
Urushiol resin remains stable, even in dead or dried plants, and therefore is equally hazardous in the winter as in the summer. The resin can be carried by smoke if the plant is burned and infect the lungs. Ingesting any part of the plant can cause a reaction in the oral cavity and gastrointestinal tract. Urushiol even stays stable in the necrotic blister and continues to irritate the skin and can initiate new lesions.
Estimates state that 50-70% of the U.S.A. population is allergic to urushiol, and would acquire the rash on casual contact. Prolonged exposure may induce an allergy to urushiol. The reaction ranges from mild to severe, and sensitivity can develop later in life. If you are allergic to poison ivy, you are also allergic to poison oak and poison sumac, and may be allergic to cashew nut shell oil, mango fruit peels, and Japanese lacquer. A person is not allergic to poison ivy the first time they touch it, but can become allergic as the skin sensitizes to the resin.
http://waynesword.palomar.edu/ww0802.htm - This link leaves my site to another fine scientific reference, with more detailed information about urushiol.
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