Hives Definition
Hives (also known as urticaria or nettle rash) are the sudden appearance of pale red lumps or weals on the skin as a result of the body's allergic reaction to a specific chemical. These weals range in size from 1 mm to many centimeters in 'giant urticaria,' and are often itchy. They are induced by vasoactive mediators, primarily histamine, which are produced by mast cells. These skin eruptions can arise anywhere on the body, creating itching that can be severe and difficult to tolerate at times.
In the great majority of instances, the weals are temporary, lasting only a few hours in one location before reappearing in another. This means that most rashes move over the body, which is a useful clinical history indicator that the rash is urticarial.
Once the allergic reaction has been triggered, the body releases a biological substance called histamine in the tissues contributing to the dilating of the blood vessels, making their walls extremely thin and permeable. The swelling of the skin that’s caused by this allergic reaction is referred to as edema.