Updated May 27, 2011.

Hives are caused by a complicated chain of events that result in the release of histamine into the surrounding tissue. Histamine causes the blood vessels to be leaky so fluid accumulates in the skin. Depending on the amount of fluid that leaks and the part of the skin it leaks into, hives can be thick and "spongy" like the previous picture. Or they can be relatively flat like these hives.