Food Allergy & the Immune System
Before we can discuss allergy and its nuances, let’s talk about our body’s defense – the immune system. Understanding how this complex system works is the key to grasping the mechanics of allergy, sensitivity and intolerance.
Inflammation and the Immune Response
Our blood has two main cellular components. The oxygen carriers are the red blood cells and the immune response team are the white blood cells.
Our white blood cells are formed in the bone marrow and other lymphoid tissues. They are mobilized to identify, destroy and remember any materials that are not part of the body.
Inflammation happens when any tissue gets injured. This could be from a cut, a harmful food, a toxin in the air or exposure to a pathogen such as a bacteria, virus or parasite. A dramatic orchestration of events occur in the tissue:
- The body causes a rush of blood to the tissue causing it to swell and swarm with different types of white blood cells to wall off the damage and clean it up.
- The tissue releases substances such as histamine, bradykinin, serotonin and prostaglandins to help destroy and repair.
Innate Immune Response
The vanguard of the immune system is the innate immune response. This is the part of our immune system that we get from our parents. It does things like create mucous or elicit a fever as an early response to an invader. This happens while the body is calibrating the acquired immune response to specifically seek and destroy the invader while creating a record to facilitate a faster future response if we get exposed again in the future.
Antigens and Allergens
Toxins and substances foreign to the body are called antigens. These large molecules are usually proteins or complex polysaccharides. If an antigen causes too robust an immune response in the body, it is called an allergen.
Lymphocytes and Antibodies
The type of white blood cell that plays a huge part of this story is the lymphocyte. These cells get created in the bone marrow and thymus. They tend to hang out in lymphoid tissues, such as lymph nodes, or specialized lymphatic tissues such as the tonsils.
The vast majority of lymphocytes are in the digestive system, as it is the largest organ being exposed to allergens. The throat and respiratory systems also have dense populations of lymphocytes. When an antigen gets in the body, it gets carried off into the lymphatic ducts and lymph nodes. Macrophages engulf it and then present it to the lymphocyte to be destroyed.
Lymphocytes that mature in the thymus are called T lymphocytes (T-cells). We eat and inhale millions of antigens and each one has a T lymphocyte to respond to it. This is where the body destroys that which is “nonself”.
The lymphocytes that mature in the bone marrow are called B lymphocytes (B-cells). When a B lymphocyte gets presented with an antigen, it swells and changes into a plasma cell that can now secrete large destructive proteins called antibodies. These antibodies can then bind to the antigen and facilitate its destruction. Another name for antibodies are Immunoglobulins.
Immunoglobulins
There is an immunoglobulin expressed for every antigen we ever encounter. They fall into one of five classes: IgM, IgG, IgA, IgD and IgE. By far IgG immunoglobulins are the most common in the body, and IgE by far the least.
What is Food Allergy?
The body’s immune system is uniquely designed to identify and fight off bacteria and viruses that can make us sick. But what happens when this system begins to attack innocuous things like the food we eat? According to the American College of Allergy, Asthma and Immunology, food allergy is when a person’s immune system identifies a food or an ingredient in a food as a threat to the body. As a result, the body triggers an auto-immune response (or reaction) to protect it.
While there are other adverse reactions that include metabolic, toxic, pharmacologic, and undefined mechanisms; true allergy involves an IgE-mediated response to antigens. IgE is significant because it has a high affinity for mast cells and basophils. When an antigen gets exposed to the IgE bound to the mast cell or basophil, it causes it to rupture and consequently histamine leaks out. Histamine causes profound vasodilation (widening of blood vessels), leading to stuffy and runny nose, sneezing and itching.
Adverse Allergic Reactions
Reactions typically involve the immediate onset of symptoms such as cough, chest tightness, wheezing, hives, rash, vomiting, diarrhea, bloating, severe swelling, and life threatening anaphylaxis. IgE -mediated food allergy can lead to an accumulation of white blood cells called eosinophils. This can lead to inflammatory digestive disorders of the stomach, small and large bowel.
In the case of anaphylaxis, the immune system floods the body with chemicals that can make the person go into shock. Their blood pressure drops and airways constrict which makes breathing difficult.
Treatment for anaphylaxis is epinephrine which rapidly increases blood pressure, dilates and prevents swelling of the airways. Epinephrine needs to be promptly administered to the patient through an auto-injector. This still needs to be followed up at the emergency department as without immediate medical attention, anaphylaxis can be fatal.
Families with members who are severely allergic need to always know where the auto-injector is and be educated on how and when to use it. To learn more about how to use one correctly, check out this page from EpiPen.
Although food and air borne allergens are most discussed here, please note that someone can be triggered into anaphylaxis by antibiotics, latex, insect stings, anesthetic, NSAIDs (non-steroidal anti-inflammatory drugs), etc.
While serious adverse reactions like anaphylaxis commonly begin in seconds upon exposure to the allergen, in the case of gastrointestinal symptoms, the reactions can be delayed for minutes to hours later. This makes finding the source of the reaction difficult.
Most Common Food Allergens
The ACAAI lists the top 9 food allergens to be eggs, dairy, peanuts, tree nuts, fish, shellfish, wheat, soy and sesame. While some individuals need to ingest the offending allergen, others are so allergic that they can react from touching the food or in extreme cases, develop symptoms upon exposure to fumes from triggering foods being cooked (e.g., fish).
While food allergy can develop at any time of a person’s life, it commonly begins in young children. According to Food Allergy Canada more than 500,000 Canadian children under the age of 18 have diagnosed food allergies. While it is possible to outgrow some allergies like milk or egg; peanuts, tree nuts and shellfish tend to be lifelong.
Identify Triggering Foods Through A Food Diary
The simplest step to take when a food allergy is suspected is to keep detailed records of the reactions.
- Keep an ongoing daily journal of meals and snacks that the person eats throughout the day. Include notes as to how it was prepared (raw, cooked).
- Include dates and times and duration of any allergic reaction and any medication administered.
- Take photos or video of rash or swelling so that it can be shown to an allergist later for diagnostic consideration. Using a smartphone to record this information has the added benefit of providing a date, time and location stamp to each file which makes it easier to find and compile for future appointments.
Food Allergy Testing
There are a wide array of diagnostic tools for assessing IgE mediated allergy:
- Elimination diet – This involves removing certain foods for a period of weeks and then follows a monitored re-introduction of the food to see if allergic symptoms reoccur. This should only be attempted with allergies that are considered mild in severity.
- Oral food challenge is considered the gold standard for food allergy diagnosis. It consists of administering a suspected food in a protected clinical setting to observe for adverse response. This is often done to see if a food allergy that was diagnosed as a child, is still present as an adult.
- Skin Testing has been used to diagnose allergy for over a hundred years. These tests are highly sensitive and low cost. There are three common types:
- Prick puncture test (scratch test) – Commercial extracts of an allergen is introduced by a pin prick into the surface of the skin to detect the presence of specific IgE antibodies bound to mast cells.
- Prick-by-prick test is a skin prick test performed using fresh, cooked or raw food.
- Patch test – The suspected food is applied directly on the skin using special supports and removed after a few days to study IgE or cell-mediated responses. This is often used to measure allergic dermatitis.
- Blood Testing
- Total serum IgE blood – This test measures the total concentration of IgE in the blood but does not identify specific triggers.
- RAST – “Radioallergosorbent” test is a fluorescent enzyme-labeled antibody analysis that measures absolute IgE levels.
- CRD – “Component Resolved Diagnosis” is a test that utilizes purified native or recombinant allergens to detect specific IgE antibodies against individual allergenic molecules.
- BAT – “Basophil Activation Test” measures the expression of activation markers on the surface of basophils. It has the capacity to predict the severity of allergic reactions.
Non-IgE-Mediated Food Allergy
What happens when the tests come back negative for definitive allergy? Many people suffer adverse reactions to foods despite testing negative for an IgE-mediated allergy. Some of these conditions include celiac disease, food protein-induced enterocolitis syndrome, food protein-induced enteropathy, and food protein-induced allergic proctocolitis.
These types of food allergy are harder to diagnose as there is a delay of the onset of symptoms from hours to days, which renders skin-prick type testing useless. Due to the wide range of symptoms there are often overlaps in specialist fields such as Immunology, Gastroenterology and Dermatology.
You can read more in depth about these medical conditions in this article by Domenico Gargano.
The Importance of Reading Food Labels
The best way to treat food allergy is complete avoidance of the allergen. Even trace amounts can produce serious reactions in allergic people. Patients and caregivers need to remember to carefully read ingredient labels on food packaging before consuming or serving to an allergic person.
Although companies are often required by law to list all ingredients in a product, there is a grey zone in labelling. Companies can use “natural flavour” as an ingredient which could be problematic to some. In Canada “May Contain” and other allergen warnings are only mandatory if they are one of the 11 identified priority allergens: cow milk, egg, peanut, tree nuts, wheat, soy, fish, shellfish, sesame, mustard, and sulphites.
If reactions are happening despite the strict avoidance of allergic foods; cross-contamination or unintentional exposures may be the culprit. This is particularly important to keep in mind while dining out. Read Allergy Canada’s resource on measures to take to mitigate risks of food allergen exposure in food establishments.
If venturing into restaurants, alerting the kitchen staff to any allergens before you order is a crucial step in allergy management. Telling them upfront that you carry an auto-injector sends the message that it is a serious matter and they are more likely to provide a higher level of care.
Our family has opted on occasions to bring pre-made meals with us to restaurants when we thought that there was a risk of exposure. That way we can all enjoy the social aspect of eating out together while taking the stress and worry of a severe reaction out of the picture.
Cross-reactivity in Food Proteins
Pea protein – a common ingredient in processed vegan foods – has a similar protein structure to peanuts. They are both in the legume family that grow on the ground. People who are allergic to peanuts can have a similar reaction to pea protein, albeit usually not as severe.
It is interesting to note that people may not react to peas in natural form but will react to pea protein in the commercial form. This is because the protein has been extracted and is much more concentrated.
So, reading the label of a “peanut-free” protein bar, veggie burger or dairy-free cheese won’t cut it. The pea protein in these products could still cause problems for some allergic individuals.
Coping With a Food Allergy Diagnosis
Food Allergy Canada estimates that 1 in 2 Canadian households are impacted by food allergies. There are plenty of resources available to people who need help navigating risks. Here are some adaptations to consider making at home, school and workplace to stay safe and reduce risk from allergic food reactions.
- Remove the allergen from your home to mitigate the risk of exposure. If this is not possible, be careful to use separate utensils and equipment in food preparation and wash surfaces thoroughly.
- Carefully read all food product labels before consuming them or serving them to an allergic person.
- Make allergen free meals and snacks from scratch to ensure there is no exposure.
- Submit up-to-date medical forms at school to notify teachers, staff and students about your child’s allergy.
- Wear medical alert bracelets and necklaces to inform others of an allergy in case of emergency.
- Make sure that allergy medication (most importantly an auto-injector) is accessible at all times and that others in the school or workplace know where to find them.
- Use food allergy alert tags and labelling on school bags and lunch boxes.
