Most people develop a rash, blisters, and itching after contact with them.
Cases usually occur in the spring and summer.
Poison ivy and oak have three leaves on each stem and grow as a vine or bush. Sumac has two rows of leaves opposite each other and one leaf at the end of the stem. It grows as a bush or tree. The leaves of all three plants are shiny and coated with an oily chemical, which causes the allergic reaction. The oils are also in the stems and roots of these plants.