Poison ivy: Skin inflammation resulting from contact with oils from the poison ivy vine. Chemicals produced by this vine cause an immune reaction, producing redness, itching, and blistering of the skin.

Poison ivy (Rhus radicans) is a woody shrub or vine. It is found throughout the US and many other areas of the world. Red-tinged leaves grow in groups of three. The leaves may have smooth, fine-toothed, or lobed margins.

The allergen in poison ivy is predominantly 3-n-pentadecylcatechol. It is found in the resinous sap material produced by the leaves, fruit, stem, and bark. Poison ivy is usually considered a summer problem in the US but it can and does occur year-round.

Treatment usually involves topical medication, such as calamine lotion, but in severe cases cortisone medications are sometimes given by mouth.