Urticaria refers to a group of disorders affecting adults and children, in which red patches and weals occur in the skin. The release of chemicals such as histamine from mast cells in the skin causes small blood vessels to leak and results in tissue swelling. The weals can be a few millimetres or several centimetres in diameter, coloured white or red, often surrounded by a red flare, and frequently itchy. Each weal may last a few minutes or several hours, and may change shape. Weals may be round, or form rings, a map-like pattern or giant patches.

The surface weals may be accompanied by deeper swelling of eyelids, lips, hands and elsewhere. The swelling is called angioedema. Angioedema may occur with or without urticarial weals (10%).

Rarely, urticaria results from an autoinflammatory disease such as systemic lupus erythematosus or Schnitzler syndrome, or an inherited condition such as Muckle-Wells syndrome or cryopyrin-associated periodic syndrome.

Generalized ordinary urticaria (hives) presents with spontaneous weals anywhere on the body. It is often classified according to how long it has been present.

• Acute urticaria is of recent onset (hours, days or a few weeks).
• Episodic urticaria describes intermittent attacks of urticaria, which may last a few days or a few weeks.
• Chronic urticaria has persisted for several months or years.

Urticaria may not be present all the time. Some find it more noticeable at certain times of day, or when they are warm or emotionally upset.

Acute urticaria is sometimes due to allergy. Allergy depends on previous exposure to the material, and the development of an immune reaction to it. An immunoglobulin called IgE is involved, which attaches itself to a receptor on the mast cell and causes it to release its chemical mediators.

The cause of an allergy may be:

• Medicine: most often an antibiotic, but many other drugs have been reported.
• Food allergy: tiny amounts of fish, eggs, nuts and kiwifruit (many others have been reported less often).
• Bee or wasp stings.
• While most allergies involve ingestion,injection or inhalation of the allergen, sometimes allergic urticaria can result from skin or mucosal contact with an allergen e.g. rubber latex.

Most allergies are mild, but very allergic individuals may develop serious anaphylactic shock within a few minutes of exposure. The most frequent causes are antibiotic injections, bee stings or ingestion of peanuts. Anaphylaxis results in urticaria, a tight chest, wheezing, faintness and collapse. Medical attention must be sought urgently. A subcutaneous adrenaline (epinephrine) injection will usually be given. Those prone to anaphylaxis should carry an emergency supply (e.g. an EpiPen™).

Most cases of urticaria are NOT due to allergy. Histamine and other vasoactive chemicals can be released into the skin for many reasons. In these cases urticaria can occur the first time that a person is exposed to the material.