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Urticaria — commonly called hives — are red raised bumps or welts on the skin that can occur anywhere on the body, usually as a reaction to an allergen (something that causes an allergic reaction).

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The red welts of hives arise when mast cells in the bloodstream release the chemical histamine, which causes tiny blood vessels under the skin to leak. The hives can appear as small individual spots or large interconnected bumps.

Most often, hives are associated with an allergic reaction (to things like food, medications, pets, and insect bites), and can occur within minutes of exposure.