Contact dermatitis. Poison ivy, oak and sumac all contain the same substance which causes a severe contact skin reaction in most people. The rash will appear from about 24-48 hours after the exposure to the plant oil. It is red, raised, very itchy and often has a seepy discharge or blisters. It will often appear in "streaky" lines on the areas which were exposed. Rash on the face can cause a lot of swelling.