Poison ivy ( Rhus radicans ) is a woody shrub or vine found throughout the United States .  Red-tinged leaves grow in groups of three and have smooth, fine-toothed or lobed margins, and small, yellow-green flowers form cream-colored fruit.  The allergen, predominantly 3-n-pentadecylcatechol, is found in the resinous sap material uroshiol, which is produced by the leaves, fruit, stem and bark.  Therefore, although delayed-hypersensitivity dermatitits induced by contact with poison ivy is usually considered a summer problem in the northeastern United States , it can (and does) occur year-round.

Adapted from The New England Journal of Medicine. Vol. 347. July 4, 2002