ivy always grows with groups of three glossy, pointed leaflets. 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 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. After being absorbed by the skin, it is recognized by the immune system's dendritic cells, otherwise called Langerhans cells. These cells then migrate to the lymph nodes, where they present the urushiol to T-lymphocytes and thus recruit them to the skin, and the T-lymphocytes cause pathology through the production of cytokines and cytotoxic damage to the skin. This causes the painful rash, blisters, and itching. Once this response starts, only a few treatments, such as cortisone or prednisone, are effective. Medications that can reduce the irritation include antihistamines (diphenhydramine (Benadryl) or cetirizine (Zyrtec)). Other treatments include applying cold water or calamine lotion to soothe the pain and stop the itching. Mechanism of action 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 a molecule 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. See also Anti-itch drugs, 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. References Further reading External links Poison Oak at Wayne's Word Reduce poison ivy dermatitis with the following steps –