Hives also known as Urticaria

Urticaria (or hives) are a kind of skin rash notable for dark red, raised, itchy bumps. Hives are frequently caused by allergic reactions, however there are many non-allergic causes. For example, most cases of hives lasting less than 6 weeks (acute urticaria) are the result of an allergic trigger. Chronic urticaria (hives lasting longer than 6 weeks) are rarely due to an allergy. The majority of patients with chronic hives have an unknown (idiopathic) cause. Perhaps as many as 30-40% of patients with chronic idiopathic urticaria will, in fact, have an autoimmune cause. Acute viral infection is another common cause of acute urticaria (viral exanthem). Less common causes of hives include friction, pressure, temperature extremes, exercise, and sunlight. It may be true that hives are more common in those with fair skin.

Angioedema is a related condition and if you would like to read further on Urticaria, the following link is a good read.