There are three major groups of skin diseases caused by allergy: eczema, allergic contact dermatitis and hives. Eczema occurs most commonly in children and involves red, thickened, swollen patches of skin on the cheeks, scalp, neck and trunk that itch and can be painful. Allergic contact dermatitis describes the onset of a rash, swelling, blistering or other effect after the skin touches an irritating substance such as clothing materials and dyes, latex, cosmetics, soaps, perfumes or certain plants like poison ivy. Sometimes the reaction is triggered by exposure to ultraviolet light (called photoallergy), for example putting on sunscreen and stepping outside. Hives, also called urticaria, are itchy rashes with bumps resembling insect bites. They can occur in small patches or all over the body, and last anywhere from a few minutes to several months.