The wheal and flare response in allergy testing is primarily due to the release of?

The wheal and flare response observed during allergy testing is primarily due to the release of histamine from mast cells in the skin. When an allergen is introduced, it binds to IgE antibodies that are already bound to mast cells, leading to the degranulation of these cells and the subsequent release of histamine.

Histamine acts as a potent vasodilator, increasing the permeability of blood vessels in the area of allergen exposure, which causes the characteristic redness (flare) and swelling (wheal) in the skin. This immunological reaction is a part of the body's hypersensitivity response and is crucial for the immediate allergic reactions observed in conditions such as hay fever or atopic dermatitis.

While serotonin, cytokines, and prostaglandins play roles in various immune responses, they do not primarily cause the immediate wheal and flare reaction specific to allergy testing. Therefore, histamine is the key mediator responsible for this classic allergic response.