The skin condition we call hives is referred to in medical terms as urticaria. A hive, or wheal, is a symptom of urticaria. However, most people do not develop one hive when they have urticaria, hence the name hives.
Urticaria is a common condition that can affect any person of any race at any age in any season of the year. It occurs in up to 20% of the population at one time or another. Hives can occur on any skin surface, but usually spare the palms and soles of the feet. Hives are classified as acute or chronic depending on the length of the episode.
A hive, or wheal, is a circular, red, spongy lesion that evolves and changes over minutes to hours. It is usually surrounded by an area of redness called a flare. Hives can vary in size from a few millimeters to giant hives covering a whole extremity.
Hives result from dilation of capillaries allowing fluid to leak out into the surrounding tissue, the epidermis. They resolve when the body absorbs this fluid. The border of a hive is described as polycyclic, or made up of many circles, and changes as fluid leaks out and then is absorbed. Pressing on a hive causes the skin to blanch distinguishing it from a bruise or papule.
A condition called angioedema occurs when capillaries dilate and leak fluid deeper into the skin, into the dermis and subcutaneous tissue. Angioedema typically causes swelling of the lips, larynx (producing hoarseness or shortness of breath), or the lining of the stomach and intestines (causing abdominal pain). Many times angioedema occurs with an episode of hives.
Hives are itchy because the swelling occurs in the epidermis, which has many nerve endings. The intensity of the itching varies from person to person and episode to episode. Angioedema typically does not itch since it involves swelling in deeper structures where there are fewer nerve endings.
Hives and Histamine
Hives occur in response to a complicated chain of events that leads to the release of a chemical called histamine into the skin. Histamine is located in certain white blood cells called mast cells, which are most abundant in the skin around capillaries. If properly triggered, these mast cells release granules of chemicals, the most powerful of which is histamine. Histamine causes the cells making up the blood vessels to contract allowing fluid to leak out of the blood vessel. Red blood cells are too large to leak out of these "holes". Injecting histamine into the skin causes a triple response of redness, leaking of fluid producing a hive, and the flare or redness around the hive.