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Urticaria - Hives
Hives , or "wheals", are pale red swellings of skin that occur in groups on any part of the skin. Urticaria is the medical word for hives. Each hive lasts a few hours before fading without a trace. New areas may develop as old areas fade. They can vary in size from as small as a pencil eraser to as large as a dinner plate and may join together to form larger swellings. Hives usually are itchy, but may also burn or sting.
Hives are formed by blood plasma leaking out of small blood vessels in the skin. This is caused by the release of a chemical called histamine. Histamine is released from cells called "mast cells" that lie along the blood vessels in the skin. Allergic reactions, chemicals in foods, or medications can cause histamine release. Sometimes it's impossible to find out why hives are forming.