Definition. Wheals are solid, circumscribed, variously sized and shaped, edematous elevations of the skin, usually pinkish white in color and of an evanescent character.
Wheals are pin-head- to bean-sized, occasionally larger, and are commonly firm and usually flatly convex or hemispherical; but may occur in circles, bands, gyrations, or coalesce and form irregular patches. Generally, they are of a light pink color with a whitish center, and sometimes present a pink or a white anemic areola. Wheals are rarely persistent. One or many are-evolved in crops or successively in a few minutes or even in the fraction of a minute, and disappear as rapidly at the end of a few hours or days. They may become purple from hemorrhage, be converted into bullae or occasionally leave behind pigmented macules or other lesions. They are always accompanied with marked sensation of stinging, tingling or itching.
Wheals originate from angioneurotic irritation, which causes a sudden exudation of serum from the blood-vessels into the papillae of the corium. Contraction of the vessels produces an anemic center, and, at the same time, prevents resorption. Relaxation of the capillaries is followed by rapid absorption. Wheals are diagnostic of urticaria but they may be caused by local influences, such as the stings of insects and contact with the ordinary nettle. They have often been observed after hypodermic injections of watery solutions.