Characteristics
Urushiol is a pale-yellow liquid with density of 0.968 and a boiling point of 200 °C (392 °F). It is soluble in alcohol, ether, and benzene. 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 souce. Whereas western poison oak urushiol contains chiefly catechols with C17 side-chains, 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 reacts with the saturated urushiol alone, but over 90% react with urushiol containing at least two degrees of unsaturation (double bonds). Longer side chains tend to produce stronger reaction. This suggests that the allergic contact dermatitis is caused by Van der Waals' forces.
Before urushiol has been absorbed by the skin it can be removed with soap and water. However, time is of great importance as 50% of urushiol is absorbed within 10 minutes. Once urushiol has penetrated into the skin, attempting to remove with water is inefficient. Once urushiol has been absorbed by the skin it is recognized by dendritic cells called langerhans cells that are part of the immune system. These cells then migrate to 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 by producing cytokines and cytotoxic damage to the skin.
Urushiol is an oleoresin contained within the sap of poison ivy and related plants, and after injury to the plant or during late fall, the sap leaks to the surface of the plant where the urushiol becomes a blackish lacquer after contact with oxygen at a certain temperature and humidity.
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