Hives are red, itchy, slightly elevated swellings. Itching may be severe. Hives have clearly defined borders and may have a pale center. Typically, crops of hives come and go. One hive may remain for several hours, then disappear, and later, another may appear elsewhere. After the hive disappears, the skin usually looks completely normal. The swelling is caused by the release of chemicals (such as histamine) from mast cells in the skin, which cause fluid to leak out of small blood vessels temporarily.