What causes allergies and food intolerances?

It is estimated the 2 million people in the UK have a food allergy and many more have food intolerances. What actually happens to someone who has an allergic reaction and what are the solutions?

The difference between an allergy and intolerance

An allergy is a reaction of your immune system that triggers symptoms like rashes, wheezing and difficulty breathing. A food intolerance is less severe because it does not involve the immune system. The symptoms are mostly limited to digestive problems and will not usually be life-threatening. 

What causes allergies? 

Allergies are caused by substances in foods (and other sources of allergies) called allergens.  Allergens have epitomes which are fragments in their antigen. These antigens are found on the cell membrane and act as identifiers. Pathogens such as viruses and bacteria have antigens too which helps our immune system recognise a foreign cell and destroy it before it causes disease.  The antigens of allergens are not harmful, but the immune system’s response to that antigen is what causes symptoms. This is because the antigen will stimulate the production of antibodies by the B cells in the immune system. These antibodies are complementary to the allergen’s antigen so they bind. 

The most common antibody involved in allergic reactions is IgE antibodies (Immunoglobulin E) but there can be other reactions known as non-IgE mediated reactions. Usually IgE antibodies create immediate reactions compared to non-IgE antibodies that can take longer for symptoms to develop. 

The IgE Reaction 

When an allergen first enters the body it is the first time the immune system has been exposed to it’s antigen. The immune B cells will produce plasma cells that produce and release IgE antibodies that are complementary to the antigen. This means the antibody can form a complex with the antigen. The IgE antibodies will then be presented on a specific type of cell called a mast cell. This is a process known as sensitisation.

This is a mast cell. It has IgE antibodies presented on it’s cell membrane which are a complementary shape to the allergen. The mast cell contains granules (the sacks in the middle) which contains chemicals that play a role in inflammation and allergic reactions. Usually, they are found in the lungs, gastrointestinal tract and skin which are common areas were symptoms of allergic reactions appear.

Sensitisation means that the immune system is now sensitive to this particular antigen and will respond to it if it is exposed to the body again. Next time a person is exposed, the mast cells with IgE antibodies present will recognise the allergen. This will activate the mast cell so that it will release a series of chemicals such as histamines.

On the left side of this photo, you can see an resting mast cell. And on the right you can see an activated mast cell that has released histamines in a process called degranulation.

Histamines and other chemicals are responsible for the symptoms experienced from an allergic reaction.

  • Histamines = vasodilation (widens the blood capillaries) to increase flow of blood and white blood cells to the area. This increases the heat of the area whilst experiencing inflammation.
  • Cytokines = increase the temperature of the area to provide better conditions for the immune cells to function. Cytokines also signal to other immune cells in a process of chemotaxis.
  • Granulocyte Macrophage Colony-Stimulating Factor = attracts white blood cells to the area.
  • Leukotrienes = cause the tightening of muscles (for example, airway muscles that make it difficult to breath) and stimulate the production of mucus.

There can also be cross reactive allergies that don’t need to sensitisation step of the IgE reaction. This is because many antigens can be similar in shape and trigger an immune response of the same IgE antibody. So people who are allergic to peanuts may also be allergic to another substance that has a similar shaped antigen so they stimulate the same antibody.

What causes food intolerances?

Food intolerances are usually caused by the absence of a particular enzyme.

An enzyme is a biological catalyst that speeds up the break down of many substances. They are complementary to a particular substrate (substrate is in yellow) and will bind to a substrate to break it.

The absence of an enzyme will mean a substance is not broken down, therefore, it will build up. Often this can cause build up in the large intestine and stimulates the production and overgrowth of bacteria. These bacteria will usually be beneficial to our gut but too many will produce an excess of substances and waste. As a result, symptoms to do with digestion will arise such as diarrhoea and gas. For example, lactose intolerance is caused by the lack of lactase (an enzyme that breaks down lactose into glucose and galactose).


The best solution is to avoid these foods or substances as much as you can. When you have a food intolerance, this becomes less of a problem because the issues usually aren’t life threatening. For food allergies this becomes more of a concern. Many people carry around EpiPens which inject a hormone called epinephrine (also known as adrenaline) into the body. During an allergic reaction, chemicals can drop the heart rate of a person to a life-threatening levels and restrict the airways. The adrenaline works by relaxing the airway muscles and raising the heart rate temporarily whilst the body recovers from the reaction.

Scientists are also researching allergen immunotherapy. This works by desensitising the body to an allergen by giving increased amounts of it to a person. The immune system will begin to tolerate it and become less sensitive to its antigen. However, this can’t work well with severe allergies yet and is still under research.

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