The Physiology of Food Allergies

Photo Oct 26, 8 44 31 AM

Food allergies are relatively common in the United States.

According to research estimates, they affect:

  • 8% of children
  • 5% of adults (because some kids grow out of their allergies)

The “Big 8” most common food allergens include:

  • Peanuts
  • Tree nuts
  • Fish
  • Shellfish
  • Cow’s milk
  • Egg
  • Soy
  • Wheat
[PS- If you’re not sure what the differences are between an allergy, sensitivity, and intolerance, check out this article.}

The first step in becoming allergic to a food is “losing oral tolerance”.

This is a fancy way of saying that your body is losing the normal (tolerant) response to the food, and going rogue.

To develop a full-blown allergic response to a food, it actually requires TWO exposures to the food.

 

On the first exposure, your body has to decide whether this new food is friend or foe. For some reason (probably largely genetic), some people have over-protective immune systems that tend to flag ordinary foods as threats.

Once the immune system has flagged the food as a threat, it starts preparing to defend itself for the next exposure.

To do this, your body’s B-cells (the type of immune cells that make antibodies) start producing antibodies against the threatening food. The type of antibody involved in allergies is called IgE (immunoglobin E) (immunoglobin is just another name for antibody).

Here’s what an IgE antibody looks like:

Screen Shot 2016-10-26 at 10.55.38 AM

Each “threatening” food gets its own unique type of antibody that is only capable of binding to that specific food antigen.

So, on the first exposure, you don’t actually feel any symptoms. Although your body knows it doesn’t like that food, it’s not ready to immediately launch an attack (that takes some time and energy, after all).

In-between the first & second exposure, your body is preparing itself for a second attack.

Once your body has made a whole bunch of IgE for the offending food, those antibodies circulate around your blood stream & attach themselves to mast cells.

Mast cells are a type of tissue-specific white blood cell, located on membranes of your body, like the skin, mucous membranes, GI tract, and lungs.

Mast cells contain packets (called granules) of pain-causing & inflammation-inducing chemicals that they store up & are ready to release into the environment at a moment’s notice. These chemicals (also known as “mediators”) are responsible for creating the unpleasant symptoms of an allergic response.

Here’s what mast cells look like when they have IgE antibodies attached to them:

mast_cells_with_ige

The IgE antibodies are kind of like soldiers, guarding your body from any threats, always on the lookout for the presence of the “threatening” food. Fun fact: one mast cell can have 100,000 different IgE molecules on its surface!

The second time you eat the offending food, your body’s immune system is ready and waiting with IgE “soldiers” mounted on mast cells. This time, you WILL have an allergic reaction.

Here’s what happens:

You eat the offending food, digest and absorb it into your bloodstream. Now those food antigens are circulating through your blood. Eventually, the food antigens will come into contact with the IgE antibodies mounted on your tissue’s mast cells.

When this happens, the IgE antibodies latch onto the food antigen and “capture” it (this is called crosslinking, because it happens when two IgE antibodies attach to the same food antigen). It’s sort of like when two soldiers capture an enemy who is trying to infiltrate their castle. “Aha!”, they say. “We’ve got you now! Time for punishment.”

The attachment of the food antigen to the IgE antibodies causes the mast cell to “degranulate”, aka release packets (granules) of pro-inflammatory molecules like histamine and cytokines, which trigger an allergic reaction.

mast_cells_degranulating

The purpose of degranulation is to destroy the threat.

The chemical mediators (like histamine) released from the mast cell cause physiological changes in your body. In fact, they are called “mediators” because they do just that. They “mediate” changes in the body.

Each mediator causes a unique physiologic change, but in general, they increase blood flow to the area and increase blood vessel permeability so that more immune cells can come to the area and attack the threat (this is also what causes redness and swelling). The mediators also increase smooth muscle contraction to help expel any allergens from the lungs or GI tract (and cause symptoms of asthma and diarrhea).

The symptoms you experience depend on which mast cells degranulate.

For example, if the mast cells in your skin degranulate, you may get hives. If the mast cells in your lungs degranulate, you may have trouble breathing or develop an asthma attack. If the mast cells in your GI tract degranulate, you may get diarrhea.

If mast cells start degranulating all over your body, you are experiencing anaphylaxis, a serious and potentially life-threatening reaction that may require an epinephrine injection to combat.

When anaphylaxis occurs, many mast cells degranulate at once, causing blood vessels all over the body to increase their permeability. This causes too much liquid to leak out into the tissues, leading to severe swelling and a dangerous drop in blood pressure that can cause your organs to stop functioning. A quick shot of epinephrine (adrenaline) is need to rapidly constrict blood vessels & bring blood pressure back up to normal.

Symptoms of a food allergy usually occur rapidly, about 5-30 minutes after ingestion of the offending food, and are typically obvious to the person they are affecting.

So, in summary, the steps of an allergic response are:

Step 1: Sensitization

  • Your immune system flags a food you’ve eaten a threat
  • B-cells produce large amount of IgE antibodies for that food
  • These IgE antibodies dock themselves on mast cells located in your tissues, and wait for the next exposure to that food
  • You don’t experience any symptoms at this time

Step 2: Allergic Response

  • You eat the offending food again
  • The food antigens crosslink to the waiting IgE antibodies on the mast cells
  • The mast cells “degranulate” and release pro-inflammatory & pro-algesic (pain causing) chemical mediators into the surrounding tissue
  • This causes the symptoms of an allergic response, like swelling, itching, trouble breathing, etc.
  • The symptoms will subside when exposure to the allergen is gone, or when anti-allergy medications, like anti-histamines, are taken.

Stay tuned for more articles on allergies, sensitivities, and intolerances. If you’d like a broad overview of how these 3 conditions differ, check out this article.

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