A wheal and flare reaction is a skin reaction which occurs in response to exposure to an allergen. This distinctive reaction is often used in testing for allergies to determine which allergens trigger a reaction in a patient. When a wheal and flare reaction occurs outside the context of a doctor's office, it can be a sign that someone is about to experience a severe allergic reaction to something he or she has come into contact with, and it can be a good idea to be prepared for further symptoms such as difficulty breathing.

There are two separate mechanisms involved in a wheal and flare reaction. The first is the wheal, which is a pale raised welt on the skin caused by a rush of serum released by mast cells, specialized cells which float around in the body looking for potential allergens and other troublemakers. The flare is caused by the dilation of blood vessels in the surrounding area, creating a halo of red, flushed skin which surrounds the wheal.

When people undergo skin testing for allergies, allergens are rubbed onto their skin and then the skin is pricked to see if a wheal and flare reaction appears. If no reaction is observed, allergens may also be injected under the skin. Injection triggers a more intense reaction, making a wheal and flare clearly visible even if someone is only mildly allergic.

You may hear this phenomenon referred to as a “wheal and erythema reaction” by some doctors. In a controlled environment such as a doctor's office where allergy testing is performed, the staff have access to bronchiodilators and other emergency medications and equipment to intervene in the event that someone experiences a severe allergic reaction. Typically when allergy testing is ordered, patients are also asked to refrain from taking antihistamines and certain other medications which might decrease the intensity of the wheal and flare reaction.

It can take two to four hours for a wheal and flare reaction to occur. Because of the long lead time, allergy testing usually involves testing for a battery of allergens all at once, to make the long wait to monitor reactions worth it for the patient. Several companies manufacture controlled doses of potential allergens for use in allergy testing, to ensure that testing is as consistent as possible. Patients who go in for allergy testing are usually alerted to the fact that they may have to spend some time waiting around for results, and bringing in a thick book is highly advised.