What causes a wheal in dermatological terms?

A wheal is a raised, erythematous area of skin that is often itchy and can appear and disappear rapidly. The underlying cause of a wheal is localized edema in the dermis, which is the layer of skin beneath the outer epidermis. This swelling occurs due to an increase in blood flow and the leakage of fluid from blood vessels into the surrounding tissue, typically as a result of an allergic reaction, insect bite, or other irritants. This is consistent with the characteristics of urticaria or hives, where wheals are commonly observed.

Localized edema in the dermis specifically leads to the swelling and distinct raised appearance of the wheal, while other factors such as irritants or infections would not directly cause this specific type of skin reaction.