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What is a Wheal and Flare Reaction?

Mary McMahon
Updated Mar 03, 2024
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A wheal and flare reaction is a skin reaction that occurs in response to exposure to an allergen. This distinctive response is often used in testing for allergies to determine which allergens trigger a reaction in a patient. When the reaction occurs outside the context of a medical professional'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 that 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 that 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 any 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.

People may hear this phenomenon referred to as a “wheal and erythema reaction” by some healthcare professionals. 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 that might decrease the intensity of the response.

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 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.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a The Health Board researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon991913 — On Jul 28, 2015

A wheal and flare reaction is the type of reaction one gets after a mosquito bite. The bump is the "wheal". The flare is the redness around it. Funny they didn't really explain it with something everyone can relate to. There is a protein in the mosquito's saliva that triggers the "type 1 hypersensitivity reaction" and its mostly the effect of histamine being released by mast cells. This is an allegic reaction from direct skin contact to the offending agent. Use of benadryl (an anti-histamine) cream/gel or spray results in quick relief. -Dr.B

By angelBraids — On May 11, 2011

I sometimes get wheals, no idea why and they don't appear often enough to give me any real trouble. The heat and itching is a nuisance but I treat them with a dab of toothpaste. It may look a bit odd but they calm down and clear up really fast.

By yumdelish — On May 11, 2011

@Windchime - If you are having these wheal and flare responses to unknown foods then I would definitely recommend you look into being tested. You never know, you could have problems with any number of other things too.

My sister was a regular for those shots, but the clinic never seemed so interested in finding out how to avoid the problem in the first place. It was awful to see her suffer like that. Her whole body would swell, sometimes to the point where she couldn't get shoes on!

Since she had the testing for food allergies she's been mostly fine, because she knows what to avoid.

By Windchime — On May 08, 2011

I have suffered from food allergies on and off for several years. I was once about to make an important presentation when the wheals and flares started. It's horrible because from that first welt you know there's no escaping.

Usually I just head for the doctors and get a steroid shot, although that's not a great long term solution and is often impractical. Maybe I should be thinking about a food allergy test?

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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