Urticaria - Hives

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 be 5 mm (0.2 inches) in diameter 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.

See complete Wikipedia article here.

For more information from the American Academy of Dermatology click here.