Poison ivy rash is allergic contact dermatitis caused by a substance called urushiol, found in the sap of poison ivy. This same substance can be found in other plants as well, notably poison oak, and poison sumac.

Urushiol is a colorless, or slightly yellow oil that oozes from any cut, or crushed, part of the plant, including both stem and leaves. Simply brushing against a plant may not cause a reaction. On the other hand, you may develop dermatitis without ever coming into contact with poison...