Urticaria or hives is a relatively common form of allergic reaction that causes raised red skin welts. Urticaria is also known as nettle rash or uredo. These welts can range in diameter from 5 mm (0.2 inches) or more, itch severely, and often have a pale border. Urticaria is generally caused by direct contact with an allergenic substance, or an immune response to food or some other allergen. Hives can also be caused by stress.