What is a Food Allergy?


This article was made possible due to an unrestricted educational grant from Sperri – Canada’s first plant-based meal replacement.

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Imagine having to treat a food that’s perfectly harmless and even healthy for most people as if it could poison you. That’s the daily reality for the estimated 2.3 to 3.1 million Canadians who are living with at least one food allergy.  

But unlike a poison, a food allergen doesn’t directly cause damage. Instead, it turns your body’s own infection-fighting system against you. It is also different than a food intolerance. A food intolerance is an adverse reaction to a food substance or ingredient. It affects the digestive system, as it is the inability to digest or absorb certain foods. 

If you have a food allergy, your body mistakenly labels something in that food (usually a protein) as dangerous. This activates the same defense system that fights off invading viruses, like the one that causes the flu. The next time you encounter that allergen, your body goes into attack mode, and you experience an allergic reaction.

Food allergies fall into two categories. The kind most people are familiar with is what’s called an Immunoglobulin E (IgE for short) mediated allergy. Your immune system makes proteins called antibodies to recognize and get rid of viruses and bacteria. IgE is one of these antibodies. If your body mistakes a food for a harmful invader, it will typically produce IgE antibodies that are tailored to recognize that specific food.

It’s IgE-mediated food allergies that can potentially cause a severe, life-threatening type of allergic reaction called anaphylaxis. IgE-mediated allergic reactions usually start within minutes of eating the food allergen, although they can sometimes occur a few hours afterward.

In non-IgE-mediated food allergies, other parts of the immune system react to the allergen. White blood cells known as T cells are thought to be the main actor in many IgE-mediated  allergic reactions to food. In this class of food allergy, reactions usually don’t begin until hours or sometimes days after eating the food.

It’s possible for one person to have both IgE-mediated and non-IgE-mediated food allergies.

Who is affected by food allergies?

Based on the results of a 2016 Canadian survey conducted by researchers at the University of Calgary and the University of Waterloo, an estimated 6.1 percent of Canadians report having a ‘probable food allergy’.(1) That’s roughly 2.3 million people. And it’s possible that number is even higher: up to 3.1 million people, according to the authors of a 2021 report from Dalhousie University. (It is worth noting, however, that more than one third of survey respondents who claimed to have a food allergy said they had diagnosed it themselves.) (2)

While we don’t yet know what causes food allergies, the tendency to develop them seems to run in families. You’re more likely to develop a food allergy if a close member of your family has a food allergy; if you already have one food allergy; or if you or a family member has eczema, asthma, or hay fever.

Food allergy is more common in babies and young children than it is in older kids and adults.  But you can still develop a food allergy in adulthood.

In some cases, children with a food allergy eventually grow out of it. This is more likely to happen with an allergy to egg or milk. 

And we now know that it’s possible to prevent food allergy in many babies who are at high risk of developing one due to factors like a strong family history. How? By starting to introduce food allergens by roughly six months of age. (3)

What are the most common food allergens?

According to Health Canada (4), the most common allergens, which are also referred to as ‘priority allergens’ in food are:

  • Eggs
  • Milk
  • Mustard
  • Peanuts
  • Crustaceans and mollusks (shellfish)
  • Fish
  • Sesame seeds
  • Soy

Which foods are responsible for the majority of severe anaphylactic reactions? (5)

  • Peanut
  • Tree nut
  • Shellfish
  • Fish
  • Milk
  • Egg
  • Wheat
  • Sesame
  • Soy

What are the signs and symptoms of an allergic reaction to food?

If you have an allergic reaction to a food, usually symptoms will come on minutes after you’re exposed to the allergen. But they can also start several hours afterward.

What should you be on the lookout for? You can experience any of the following (6) during an allergic reaction. It’s important to note that children might describe these differently; for instance, a ‘funny feeling’ in the mouth or throat.


  • Hives (angry red lumps), itching, rash or reddening of the skin
  • Swelling in the tongue, face or lips

Respiratory (breathing)

  • Coughing, wheezing, or trouble breathing or swallowing
  • Tightness in the throat or chest
  • Sneezing, stuffy nose, itchy nose or watery eyes

Cardiovascular (heart)

  • Skin turning pale or blue, dizziness, lightheadedness, or fainting


  • Nausea, belly pain or cramps, vomiting or diarrhea


  • Feeling convinced that something terrible is about to happen
  • A metallic taste in the mouth

Having symptoms from two or more of these categories—even if they’re mild—is a warning signal that you could be having a dangerous anaphylactic reaction.  This should be treated as      an emergency: call 9-1-1 immediately.

Other important facts:

  • Your symptoms might differ from one reaction to the next.
  • You can’t predict how severe your next reaction might be based on your last one.
  • Seemingly mild symptoms can worsen quickly.

How are food allergies diagnosed?

Diagnosing food allergies isn’t a simple matter. For one thing, there’s no test that can determine whether you definitely have one or don’t. If you think you or your child may have a food allergy, see your doctor, who can refer you to an allergy specialist.

Allergy specialists have the expertise needed to piece together clues from various sources to arrive at a diagnosis.

To begin with, your allergist will ask you for more details about the suspected allergic reaction, and your family’s medical history. These questions often include what kinds of symptoms you had after eating the food, how long after eating they occurred, and whether other people in your family have allergies.

Skin prick tests are another tool allergy specialists often use. (7) These are done by applying a small amount of allergen to a patch of skin on your back or arm, then making a prick in the same spot. This is done several times, each with a different allergen. It takes an experienced eye to interpret these tests. Itching and swelling in the test site for a specific allergen indicates that you may be allergic to it. But it’s important to know that you can get a positive result on a skin prick test without actually being allergic.

A special blood test may also help your specialist gauge how likely it is that you have an allergy to a specific food. First, a small amount of your blood is combined with an allergen. Next, this sample is tested for the presence of IgE antibodies that are matched to that food. Measuring the amount of IgE can help an allergy specialist gauge the probability that you have an allergy to a certain food. (7) Mind you, as with skin prick testing, you can have IgE to a food and not be allergic to it. It’s also possible to have an allergy even if no IgE is detected in the blood.

If there’s still some question about whether you have a food allergy after these tests, your specialist may suggest an oral food challenge. These are only done in doctor’s offices, clinics, or hospitals, with trained medical staff ready to immediately administer treatment if necessary. You are given a small amount of a food and watched carefully. If no symptoms develop after a specific amount of time, you’ll take a second, somewhat larger dose. If you do react to the food, this confirms that you are allergic.

How are food allergies treated?

The main strategy for managing food allergies is to strictly avoid the allergen, so you can prevent future reactions. This involves learning how to read ingredient lists on packaged foods, which means taking extra time and reading from start to finish, and not buying a product that doesn’t have an ingredient label. Some food companies may use “free from” statements such as “peanut free” and “dairy free”, so read these carefully as they do not replace the need to read the ingredient list.

Further to that, learning how to keep traces of the allergen from accidentally getting into your food during meal preparation and cleanup.

Accidental exposures can still happen, however, so it’s crucial that you take steps to prepare for the worst type of reaction: anaphylaxis. That means you need to carry a device containing a dose of an emergency medication called epinephrine at all times.

Your specialist will prescribe one of these epinephrine auto-injectors when you’re diagnosed with a food allergy. Epinephrine doesn’t stop anaphylaxis from happening. What it does do is temporarily reverse or prevent a life-threatening drop in blood pressure. This buys you time until you can get emergency medical help. Make sure you practice using your auto-injector, and make note of the expiry date so you can re-order before the medication expires.

A written anaphylaxis emergency plan can help you recognize warning signs and symptoms, and mentally rehearse when and how to use your auto-injector. It’s also a valuable tool for your family members, friends, or anyone else who might be present when you have a reaction. So give copies to people that you live with, or see regularly.

If you have a child with a severe allergy to a food like peanut, your allergy specialist may offer you the option of a treatment called oral immunotherapy. This is a way of gradually coaxing the immune system to become less sensitive to the allergen. Under expert medical supervision, a child is given a tiny amount of the allergen. If no reaction develops, that dose is continued for two weeks, then increased by small increments on a regular schedule. (8å)

While oral immunotherapy isn’t a cure, it can prevent some allergic reactions. How? By increasing the amount of allergen, it takes to trigger symptoms. That means your child is protected from accidental exposure to smaller amounts of the allergen. And for some families, this offers a life-changing peace of mind.

But even if oral immunotherapy isn’t an option, you can take some of the fear out of living with a food allergy by practicing avoidance, and being prepared if you do experience a reaction.

food allergy infographic