Urticaria or hives is a relatively common form of allergic reaction that causes raised red skin welts that range upwards from 5 mm (about a 1/4 inch) in diameter, itch severely, and often have a pale border. Urticaria is caused by direct contact with an allergenic substance (generally a plant, such as poison ivy), or an immune response to food or some other allergen. Hives can also be caused by stress.