Hives are an outward sign of a whole-body hypersensitivity reaction. Wheals or welts come and go on the skin or mucous membranes, and may itch intensely. Each typically has a whitish, raised patch of skin surrounded by a reddish halo. The welts may be tiny or large, and may be all over the body or confined to one area. The individual welts often disappear quickly, and are usually gone within 48 hours, although new ones may continue to appear for quite some time.
Hives occur when the body's capillaries and tiny veins get leaky.
Fluid escaping from some of these blood vessels gets trapped in parts of the skin and lining membranes of the body, causing the localized swelling in the hives. This leakiness can be caused by classic allergic reaction s in which histamine triggers an inflammatory response. It can also be caused by a number of other regulatory systems in the body in response to different types of triggers. This usually doesn't begin before adolescence. In these people, exercise itself is not a trigger, and they can drink water without a problem.