An Introduction to Food Allergies: What Happens Inside the Body?

In conversation with McGill’s Dr. Irah King

October 7, 2021

By Bavisha Thurairajah / Artwork by Erin Sass

Coming from a family that has no food allergies, I was really surprised to hear about how common they are in Canada. As of 2021, about one-in-two Canadian households are impacted by food allergies and more than three million Canadians have at least one self-reported food allergy. Most of these are peanut allergies, affecting about 2 in 100 Canadian children, while other priority food allergens include the well-known egg, fish, shellfish, cow’s milk, soy, and more. 

While they may be a widespread phenomenon, not many of us may be aware of the underlying science and immunological processes behind food allergies. I certainly was not. Even though I had never had to deal with reactions to food, I wanted to understand the basics of why some of us do. Why do our own immune systems react adversely to common foods in our environment? How exactly does this happen and what kind of factors could be involved? Beyond addressing our own curiosities, I think it is important for non-allergic individuals to learn about others’ food allergies because it may help us sympathize with their health concerns and be more considerate of those around us. Knowing the basics of the immunology behind food allergies can help avoid dismissive attitudes and behaviours towards the concerns of people living with allergies. Dismissive attitudes are often a problem within families or communities where food allergies are rare, and awareness might help resolve this. 

A food allergy is basically a sensitization of the body’s immune system to any type of food. Our immune system consists of a variety of different organs and cells and protects us from substances that can cause serious harm to our bodies like viruses, bacteria, fungi, toxins, and more. We can think of food allergies as “a good thing gone wrong,” as Dr. Irah King, a McGill professor from the Department of Microbiology and Immunology, explained, where the body interprets innocuous stimuli such as food as potential threats. Dr. King said that in the case of eating foods we are not allergic to, we do not have reactions because our bodies have built up an immune tolerance and “our systems remain ignorant or non-responsive.”

“We can think of food allergies as ‘a good thing gone wrong,’ as Dr. King, a McGill professor from the Department of Microbiology and Immunology, explained.”

The process starts with a primary exposure to a food that will become the allergen. This is the sensitization stage that occurs when the food is first ingested; fascinatingly, Dr. King mentioned that the initial sensitization to a food allergen can occur through skin contact, such as when one touches a peanut, and not only through eating it. It is usually a protein or several proteins in the food that the body misinterprets as potential threats. Allergists and researchers have pinpointed specific proteins in certain foods that may be problematic, such as ovomucoid and ovalbumin in egg and tropomyosin in shellfish. The epitopes, or allergenic parts of these proteins, where antibodies attach themselves are usually small in size with “varying degrees of resistance to denaturation by heat or acid and, therefore, can remain intact even after processing, storage, cooking and digestion.” This may explain why some people that are allergic to eggs can eat baked egg-products while people allergic to shellfish may get severe reactions simply from exposure to the steam released when cooking it. 

Interestingly, some individuals that are allergic to pollen can experience oral allergy syndrome (OAS), also known as pollen-food allergy, where they get an allergic reaction after eating a certain fruit or vegetable due to the similarities between the proteins found in pollen and the food. For example, individuals allergic to Birch pollen may get an itchy mouth or swollen tongue after eating an apple or celery because these foods have proteins that are similar to Birch pollen proteins. However, OAS is distinct from a “true” food allergy, and both have differing symptoms; the former can affect every part of the body, whereas OAS symptoms are usually restricted to areas around the mouth and/or throat. OAS results in a very local reaction, as opposed to a system-wide one. 

When the allergen enters the body for the first time and our immune system misinterprets a protein(s) in the food as a threat, the system is primed to develop a very specific type of response. Dr. King discussed how humans have two types of immune responses to harmful substances: Type 1 protects our bodies from bacteria, viruses, and fungi, and type 2 is our defense against parasitic or helminth (worm) infections. In the case of food allergies, it is the type 2 response that is activated during the primary exposure. With most food allergies, the type 2 response means that the body produces a type of antibody called immunoglobulin E (IgE) that is allergen-specific. These antibodies are involved in initiating the release of chemicals that cause the symptoms we associate with food allergies such as hives, abdominal pain, vomiting, and more. This describes IgE-mediated food allergies, which are not only the most common form of food allergy, but also the ones that are the leading cause of anaphylaxis. 

Dr. King explained that these immune cells and molecules travel all throughout the body to initiate reactions from multiple systems and organs, like an increased heart rate, skin flushing, diarrhea, and more. If the initial sensitization occurs on the skin, the immune cells that are activated there can travel all over the body, including the intestine, to respond during secondary exposures when the food is eaten. During subsequent exposures, what we observe is actually an exaggerated, hyperactivated type 2 response from the immune cells that were ready to react. 

Importantly, why does this happen? What makes our bodies react adversely to innocuous food stimuli? 

“The collective of bacteria that coexist with us and help educate our immune system can be altered by our diet and the external elements to which we are exposed.”

Food allergies can be caused by a multitude of factors, including genetic components. Some individuals, for instance, have genes that are mutated to overexpress the receptors that are important for stimulating type 2 responses, such that “when your type 2 immune response turns on, your receptors will actually be stimulated much more easily, causing this hyperactivated response,” Dr. King said. He also explained that the age at which a potential allergen is introduced into one’s diet can be a determining factor. Many observational studies show that the early introduction of allergens in the diet of infants significantly reduces the development of food allergies later in life. The Learning Early About Peanut (LEAP) trial, a 2015 study published in the New England Journal of Medicine, showed that the “early consumption of peanut in high-risk infants (defined as those with severe eczema and/or egg allergy) reduced the development of peanut allergy by 86% by 5 years of age.” This goes against the prior recommendation by researchers to avoid allergens in one’s diet which, according to the LEAP study, has actually contributed to the rise of peanut allergies. 

Additionally, Dr. King suggested that changes to our gut microbiome and commensal bacteria, the bacteria that live in and on us, may also play a part in food allergies, as they “have altered how our type 2 immune response is activated.” The collective of bacteria that coexist with us and help educate our immune system can be altered by our diet and the external elements to which we are exposed. He elaborated that being raised in over-hygienic environments actually decreases the diversity of our microbiome due to the general lack of exposure to bacteria. This may in turn lead to the improper education of our immune system and make us more sensitive to certain substances that are already prevalent in our environment. A good analogy to make sense of this, Dr. King said, is to think of our microbiome diversity in terms of biodiversity, where simply removing one species from the food chain can alter the entire ecosystem and impact it negatively. As a final note, he stated that eating a healthy and diverse diet may help our immune systems function better. Increasing one’s exposure to different bacteria through eating a diverse diet can then possibly address the prevalence and severity of allergic reactions. 

This short introduction to food allergy emphasizes that there are a lot of cells, systems, and processes involved in developing a reaction to food. Being aware of the science behind food allergies may not only address our curiosities about this interesting phenomenon, but also help us in making better decisions about our diet and health. 

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