Hives, also known as urticaria, are raised, itchy, red (or sometimes skin-coloured) welts on the surface of the skin. Hives can appear on any part of the skin and may vary in size from as small as a pencil eraser to as large as a plate. These welts may get bigger, spread, or join together. They can also change shape, disappear or re-appear within minutes or hours. The welts typically appear in batches or clusters with one batch fading away as a new batch appears. Each individual welt has a lifespan of around 24 hours or less.
Hives may result from an allergic reaction to a substance such as animal dander, insect bites, medications, pollen, nuts, shellfish, or other foods; however, they may also develop as a result of emotional stress, extreme cold or sun exposure, excessive perspiration, or another underlying illness.
There are three different classifications of hives (urticaria):
Acute urticaria is common and can appear at any age; but typically occurs in young adults. These hives are very itchy, red areas with white raised central circles that resemble mosquito bites. They are often caused by an allergic reaction or by a virus. Acute urticaria often goes away within 24 hours; however, it can last up to 6 weeks. Approximately 1 in 6 people will experience acute urticaria at least once in their lifetime.
When the hives are experienced for more than 6 weeks it is classified as chronic urticaria. Chronic urticaria is rarely due to an allergy and the cause is unknown in the majority of patients with chronic hives. When the cause is unknown, the condition is termed “chronic idiopathic urticaria”. Some studies have found that in approximately 30-60% of patients with chronic idiopathic urticaria there may be an autoimmune component. These hives may go away completely after a few months; but the condition can last for several years in some cases.
In physical urticaria, hives may show up in lines or odd shapes, appear on specific parts of the body, and appear and then disappear within 24 hours. There are several types of physical urticaria such as: