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Food Allergy | causes, symptoms, diagnosis, treatment, pathology | 2020

Food Allergy
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Food Allergy

Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create a personalized study plan with the exclusive articles, practice questions, and flashcards, and so much more.

Try it free today! A food allergy is a medical condition where there’s an abnormal immune reaction to some food.

Food Allergy
Food Allergy

Now, a variety of food proteins can cause food allergies, but the most common is known as the big eight, these include proteins within milk, eggs, peanuts, tree nuts, seafood, shellfish, soy, and wheat.

Food is essential to life, and normally food doesn’t cause an allergic reaction – in fact, the process that allows for that is called oral tolerance.

Let’s see 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 Peyer’s 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 cell surface to a nearby helper T cell. The protein fragments are presented by the dendritic cell using an MHC class II 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 Tcell 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.

Anergy is a bit like turning off that helper T cell so that 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.

Now, in food allergy, this process does network 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 towards 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 present shrimp proteins on their MHC class II molecules to helper T cells.

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 2 helper T cell or TH2 cells.

TH2 cells release a variety of cytokines like interleukin 4.

This interleukin 4 gets nearby B cells that also recognize the protein to switch from making IgM antibodies to making IgE antibodies that 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 the mast cells and basophils are ready for action.

Now, let’s say that the same person eats shrimp again, a few months later. That’s the second exposure that leads to a type I hypersensitivity reaction. Specifically, the IgE on the surface of the sensitized mast cells and basophils will bind to the shrimp antigen.

When two IgE antibodies that are near one another both bind to the same antigen, it’s called crosslinking. When two IgE antibodies crosslink on the surface of a mast cell or basophil it sends a signal down into the cell, which makes that cells degranulate and releases a bunch of pro-inflammatory molecules like histamine.

Causes

Food Allergy  causes, symptoms, diagnosis, treatment, pathology  2020
Food Allergy

This histamine gets into the intestinal tissues as well as into the blood. The histamine often binds to H1 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 space between cells, which causes edema and swelling of the lips.

In a severe case 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 the airways get smaller, and it makes breathing difficult.

Symptoms

All of this happens within minutes to hours of the second exposure. 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 and then get worse 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.

There’s also food protein-induced enterocolitis syndrome or FPIES for short, where the small intestine and 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 and typically occurs hours or days after exposure.

The way non-IgE mediated food allergy works are 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.

Food Allergy  causes, symptoms, diagnosis, treatment, pathology  2020
Food Allergy

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 it’s when people experience symptoms from both types.

There are two main types of tests to determine if a person has food allergies. There’s the skin test, where a tiny amount of 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. Then, there’s the blood test that determines if there are specific IgE antibodies in the blood specific to any foods.

This helps diagnose the IgE mediated type. Diagnosing the non-IgE mediated type is challenging and is mainly based on the dietary history of the patient.

The management of food allergies consists of avoiding triggering food. For milder allergic symptoms, antihistamines can help reduce allergic symptoms.

Finally, if there are signs of anaphylaxis, epinephrine, in the form of an EpiPen can be used to give an immediate dose into a muscle.

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 consists of avoiding all food that may cause an allergic reaction.

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