Urushiol is an oily mixture of organic compounds with allergenic and sensitizing properties found in plants of the family Anacardiaceae, especially Toxicodendron spp. (e.g., poison oak, Chinese lacquer tree, poison ivy, poison sumac), Comocladia spp. (maidenplums), Metopium spp. (poisonwood), and also in parts of the mango tree and the fruit of the cashew tree.
In most individuals, urushiol causes an allergic skin rash on contact, known as urushiol-induced contact dermatitis.
The name urushiol is derived from the Japanese word for the lacquer tree, . He did not classify it as a poison at first due to the speed with which its rash disappeared and Smith hypothesized that there may actually be medicinal uses for the plant.
Characteristics
Urushiol in its pure form is a pale-yellow liquid with a density of about 0.968 g/mL and a boiling point of . It is soluble in diethyl ether, acetone, ethanol, carbon tetrachloride, and benzene.
Urushiol is a mixture of several closely related organic compounds. Each consists of a catechol substituted in the 3 position with a hydrocarbon chain that has 15 or 17 carbon atoms. The hydrocarbon 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 C<sub>17</sub> side-chains, poison ivy and poison sumac contain mostly catechols with C<sub>15</sub> sidechains.
The likelihood and severity of allergic reaction to urushiol is dependent on the degree of unsaturation of the hydrocarbon 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.
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. Urushi lacquer is very stable. It is able to withstand disturbances from alkali, acid, and alcohol, while also being able to resist temperatures of over 300 °C. However, the lacquer can be degraded by UV rays from the sun and other sources.
These plants all have distinguishing features that will help in identification. Poison ivy always grows with groups of three glossy, pointed leaflets on leaves that alternate along the branch (in contrast to similar looking leaves of boxelder which has opposite leaves). Poison oak has a similar appearance, but with larger and more rounded lobes, with a light, velvety pubescence and grow in groups of 3, 5, or 7. Poison sumac grows in groups of 7 to 13 leaflets, always an odd number. The leaflets are ovate-lanceolate and glossy.
Allergic response and treatment
Before the urushiol has been absorbed by the skin, it can be removed with a washcloth using soap and water. Substantial amounts of urushiol may be absorbed within minutes. Once urushiol has penetrated the skin, attempting to remove it with water is ineffective. This causes the painful rash, blisters, and itching.
Once this response starts, only a few treatments, such as cortisone or prednisone, are effective. As of 2024, urushiol medications are not available in the United States.
Mechanism of action
thumb|A video describing the mechanism of action for [[poison ivy and other such plants containing urushiol]]
To cause an allergic dermatitis reaction, the urushiol is first oxidized to create two double-bonded oxygens on the chemical. It then reacts with a protein nucleophile to trigger a reaction within the skin. Dermatitis is mediated by an acquired immune response. Urushiol is too small to directly activate an immune response. Instead, it attaches to certain proteins of the skin, where it acts as a hapten, leading to a type IV hypersensitivity reaction.
Hydrocortisone, the active ingredient in cortisone, works to alleviate this condition by stopping the release of chemicals that cause the dermatitis reaction. Hydrocortisone itself does not react with urushiol in any way.
alt=|center|thumb|474x474px|Basic mechanism of Urushiol causing allergic dermatitis
See also
- Antipruritics, for treating the toxin.
- Bentoquatam, a barrier cream applied before exposure.
- Burow's solution which can treat the symptoms of the rash caused by urushiol.
- Ginkgo biloba and cashew, plants containing chemicals closely related to urushiol.
- Hapten, small molecules that can elicit an immune response under certain conditions.
- Mango trees, which may cause cross-reaction allergies with urushiol.