Urticaria (or hives) is a skin conition, commonly caused by an allergic reaction, that is characterized by raised red skin wheals (welts). It is also known as nettle rash or uredo. Wheals from urticaria can appear anywhere on the body, including the face, lips, tongue, throat, and ears. The wheals may vary in size from about 5 mm (0.2 inches) in diameter to the size of a dinner plate; they typically itch severely, sting, or burn, 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, but can also appear for other reasons, notably emotional stress. The rash can be triggered by quite innocent events, such as mere rubbing or exposure to cold.