Food allergy – causes, symptoms, diagnosis, treatment, pathology
24
August

By Adem Lewis / in , , , , , , , , , , /


Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create
a personalized study plan with exclusive videos, 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. 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 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 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 anergy. 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 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
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
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 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 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 cell
degranulate and release a bunch of pro-inflammatory molecules like histamine. This histamine gets into the intestinal tissue
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. 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
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 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 the triggering food. For milder allergic symptoms, antihistamines
can help reduce the 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
in avoiding all food that may cause an allergic reaction.


9 thoughts on “Food allergy – causes, symptoms, diagnosis, treatment, pathology

  1. The food allergies or allergic to any "medicine" , are because of consuming pro-inflammatory foods such as sugar-meat-animal products. These allergies are very rare in complete vegetarian population.

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