Three species of poison ivy are generally recognised; they are sometimes considered subspecies of Toxicodendron radicans:
Poison ivies can grow as small plants, shrubs, or climbing vines. They are commonly characterized by clusters of leaves, each containing three leaflets, hence the common expression "leaves of three, let it be". These leaves can vary between an elliptic to egg shape and will have either smooth, lobed, or toothed margins. Additionally, the leaf clusters are alternate on the stem. Clusters of small, greenish flowers bloom from May to July and produce white berries in the fall a few millimeters in diameter.
Urushiol-induced contact dermatitis is the allergic reaction caused by poison ivy. In extreme cases, a reaction can progress to anaphylaxis. Around 15 to 25 percent of people have no allergic reaction to urushiol, but most people have a greater reaction with repeated or more concentrated exposure.

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