Final answer:
A wheal is a type of skin lesion associated with allergic reactions, characterized by a soft, pale central swelling surrounded by a red zone, resulting from histamine release during skin testing for hypersensitivity.
Step-by-step explanation:
A wheal is a rounded or flat-topped, pale red papule or plaque that is characteristically evanescent, disappearing within 24 to 48 hours. The term is often seen in the context of allergic reactions, particularly during skin testing for type I hypersensitivity, where the injection of allergen extracts into the skin can result in a wheal and flare response. This response consists of a soft, pale central swelling (the wheal) surrounded by a red zone (the flare), typically occurring within 30 minutes due to histamine release from skin mast cells.
The wheal represents fluid leaking from local blood vessels into the skin, while the flare is due to the dilation of blood vessels leading to increased blood flow and redness. Recognizing and understanding the wheal and flare response is crucial in allergy skin testing and identifying the specific allergens responsible for a patient's allergic reactions.