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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.

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