Food allergy

Definitions & Key takeaways

A food allergy is an immune system response to a food that the body mistakenly believes is harmful. When someone has a food allergy, the body's immune system produces antibodies called IgE (immunoglobulin E) in response to proteins in the food. These antibodies attach themselves to cells that line the respiratory tract, digestive tract, and skin.

When the person eats or inhales the food containing those proteins, the IgE antibodies trigger the release of histamine and other chemicals. These chemicals cause allergic symptoms such as wheezing, chest tightness, nausea, vomiting, cramps, diarrhea, itchy skin or throat, swelling of lips, tongue, and eyes, or anaphylaxis. Food allergies can be life-threatening, so it's essential to be aware of them and take steps to avoid exposure to the offending food.

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Introduction0:00–0:28

A food allergy is a medical condition where there's an abnormal immune reaction to some foods. Now, a variety of food proteins can cause food allergies, but the most common are known as the big eight.
These include proteins within milk, eggs, peanuts, tree nuts, seafood, shellfish, soy and wheat. Food is essential for life and normally food doesn't cause an allergic reaction.

Physiology0:28–2:03

In fact, the process that allows for that is called oral tolerance. So let's break down how that works.
Normally, when food travels through the stomach and intestines, the proteins within them are broken down by gastric acid and proteases into tiny fragments called oligopeptides, small strings of amino acids.
These oligopeptides reach the payers patches which are bits of tissue along the intestinal wall where M cells live. M cells are intestinal epithelial cells in the gut that grab protein fragments from the intestines and then transfer them to antigen presenting cells like dendritic cells, dendritic cells present them on their surface to a nearby helper T cell.
The protein fragments are presented by the dendritic cell using an MHC class two molecule which is basically a serving platter for the helper T cells.
The helper T cell is key because it largely controls the immune response. Now, here's the catch, even if a helper T cell binds to that oligopeptide.
Another type of T cell called a regulatory T cell can release cytokines. So that the helper T cell undergoes energy.
Energy is a bit like turning off that helper T cell. So it doesn't induce an immune response.
In other words, regulatory T cells release cytokines in the lining of the intestines to help prevent the helper T cells from ever getting stimulated by food.

Pathology2:03–3:33

Now, in food allergy, this process doesn't work properly. An allergic reaction towards food happens in two steps, a first exposure or sensitization and then a subsequent exposure, which usually gets a lot more serious.
So let's say a person eats shrimp for the first time. And for some reason, the helper T cell is able to generate an immune response toward the proteins within shrimp.
Now, once shrimp proteins reach the small intestines, M cells grab the shrimp proteins and transfer them to dendritic cells that start to present shrimp proteins on their MHC class two molecules to helper T cells.
The helper T cells that recognize this antigen will bind to it with their T cell receptors. And based on the cytokines that are floating around, they mature into type two helper T cells or th two cells, th two cells release a variety of cytokines like interleukin four.
This interleukin four gets nearby B cells that also recognize the protein to switch from making IgM antibodies to making IgE antibodies, which are specific to the shrimp protein.
These shrimp specific IgE antibodies are able to attach to the surface of mast cells and basophils. At this point, the sensitization phase has come to an end and mast cells and basophils are ready for action.

IgE mediated food allergy3:33–5:21

Now, let's say that that same person eats shrimp again a few months later, that's the second exposure that leads to a type one hypersensitivity reaction.
Specifically the I GE on the surface of the sensitized mast cells and basophils will bind to the shrimp antigen when two I ge antibodies that are near one another, both bind to the same antigen, it's called crosslinking.
When two IgE antibodies cross link on the surface of a mast cell or basophil, it sends a signal down into the cell which makes that cell degranulate and release a bunch of proinflammatory molecules like histamine.
This histamine gets into the intestinal tissue as well as into the blood. The histamine often binds to h one receptors, stimulating free nerve endings near the skin which causes hives and itching.
It also causes blood vessel dilation and increases the permeability of the walls of the blood vessel which causes fluid to leak out into the space between the cells, which causes edema and swelling of the lips.
In a severe case, this can cause so much fluid to leak out of the vessels that it leads to hypotension, histamine can cause the smooth muscles around the bronchi in the lungs to contract, which means that the airways get smaller and it makes breathing more difficult.
All of this happens within minutes to hours of the second exposure. And when all of these symptoms occur and breathing is affected, it's called anaphylaxis.
And sometimes there can be a biphasic reaction which is when the symptoms improve, but then worsen again over the course of a few hours.
Now, there's also a non IgE mediated food allergy, which includes a range of gastrointestinal food allergies that affect the gut.

Non-IgE mediated food allergy5:21–6:39

There's also food protein induced enterocolitis syndrome or FPs for short where the small intestine and the colon get inflamed mostly due to cow's milk in infants.
Other causes include celiac disease where people who are allergic to gluten, have their small intestine inflamed and damaged non IgE mediated food allergy doesn't involve IgE antibodies.
Obviously. And typically it occurs hours or days after the exposure.
The way non IgE mediated food allergy works is still not well understood, but it's believed that since the gut's immune system is still maturing early on, it tends to overreact towards some food proteins like the ones found in cow's milk.
The symptoms of the non IgE mediated type are mainly gastrointestinal ones like abdominal pain, reflux constipation and loose stools.
Finally, some people develop mixed IgE and non IgE mediated food allergies and therefore they experience symptoms from both types.

Diagnosis6:39–7:22

There are two main types of tests to determine if a person has food allergies. First, there's the skin test where a tiny amount of the food, the person is allergic to makes contact with a wound on the skin.
If an allergic reaction occurs, a red itchy bump appears on the skin. Second, there are blood tests that can determine if there are antibodies in the blood that are specific to any particular foods.
This helps diagnose the IgE mediated type diagnosing. The non IgE mediated type is much more challenging and it's mainly based on the dietary history of the patient.

Treatment7:22–7:45

The management of food allergies consists mainly of avoiding the triggering food for milder allergic reactions. Antihistamines can help reduce the allergic symptoms.
Finally, if there are signs of anaphylaxis, epinephrine in the form of an EpiPen delivered straight into a muscle can be used to give an immediate dose.

Review7:45–8:18

All right. As a quick recap, a food allergy is a medical condition in which there's an abnormal immune reaction to certain food proteins.
The most common form of food allergies is IgE mediated, but there are other frequent types like non IgE mediated and mixed, which is a combination of the two.
The diagnosis consists of a skin test and a blood test. The primary management of food allergies is basically avoiding all foods that may cause an allergic reaction