poison ivy poison oak and poison sumac are plants that contain an irritating oily sap called urushiol. urushiol triggers an allergic reaction when it comes into contact with skin resulting in an itchy rash which can appear within hours of exposure or up to several days later. the rash is a form of allergic contact dermatitis. (dermatitis is swelling and irritation of the skin. )
what does a rash from poisonous plants look like?