Wheal and Flare

It is a two-stage allergic reaction found in type 1 hypersensitivity. Acute-phase mediators are chemicals that regulate the first stage, often known as the wheal. Late-phase mediators are chemicals that regulate the second stage, known as the flare. When an atopic person is given an allergen, they will have the typical local cutaneous reaction, which includes a raised, blanched wheal surrounded by a spreading “flare” of erythema, within a few minutes. This reaction is brought on by mast cells releasing histamine. Usually, this happens within an hour of being exposed to an allergen. In addition to the allergens in question, skin testing is also carried out using histamine as a positive control, which should always result in a skin reaction, and saline as a negative control, which should not result in a reaction.