Contact with poison ivy, oak, or sumac plants can cause a rash (allergic contact dermatitis). The rash is caused by contact with the oil (urushiol) in these plants. Urushiol is an allergen, so the rash is actually an allergic reaction to the oil in poison ivy, oak, or sumac.

• The leaves, stems, vines, twigs, berries, and roots of these three plants contain urushiol, even after they have died.
• Urushiol is a colorless, odorless, sticky oil that is easily spread before a rash develops.
• The rash can also occur after indirect contact with this oil. This may happen when you touch clothing, pet fur, sporting gear, gardening tools, or other objects that have come into contact with one of these plants.

No one is born allergic to any of these plants. You become allergic to them through contact with them (or with other plants). After you have been in contact with one of the plants one or more times, your body's immune system may recognize urushiol as an allergen and you may have an allergic reaction. You may be more or less sensitive than other people to the plants. A more sensitive person will react after contact with only a small amount of urushiol and have a more severe reaction.