Urushiol, an oily resin found in the leaves, stems, and roots of the plant, causes the allergic reaction to poison oak.
When this oil comes into contact with your skin, it triggers an immune response in most people, resulting in the characteristic itchy, blistering rash. You don’t need to be near the plant to have a reaction. Urushiol can stick to clothes, tools, or pets and get on your skin later.