The Science

Primary definition: The immune system reacts to a usually harmless substance known as an allergen.​

CAUSES

Allergies are caused when certain foods initiate the production of antibodies called Immunoglobulin. These antibodies travel to cells and release chemicals; symptoms of which can be felt in the nose, throat, lungs, sinuses, ears, stomach lining or skin. (AAAAI)
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No concrete cause of allergies or intolerance has been discovered as of now. There are multiple theories, however none of them have been entirely proved. A mainstream theory for the causes is the ‘Hygiene Hypothesis’. The idea is that due to the lack of early exposure to infectious diseases, parasites and microorganisms, in drinking water and modern environments the immune system is suppressed from developing naturally and thus becomes more sensitive. The idea of too much protection can then be harmful later on. Other theories include high fat diets, antibiotics, and the use of baby formula as opposed to breast milk. (ScienceLife)

High Fat Diets: People suffering from allergies have abnormal proportions of unsaturated fatty acids in their blood. After a study was carried out, it was shown that children who have allergies consumed more margarine and less butter, suggesting that perhaps an imbalance of saturated and unsaturated fats could cause an increased risk of allergies. In addition, a recent study of linking overweight children and increased risk of allergy and asthma risk was carried out. It compared the ‘meditarranean diet’ (Spain, France, Italy and Greece) with those in the UK, Australia and Ireland. The latter countries had a higher risk, further proposing that a higher overall fat content in the diet could impact allergy risk. (HealthEngine)

Breast Milk: It has been theorised that bottle feeding babies make them more prone to allergies, heart diseases and bowel issues as they grow up. Stability and health of the gut microbiome is an area which needs to develop, and the diet of a baby can heavily influence this. Breastmilk is known to be vital in growing necessary bacteria within the gut of a baby which will help with the digestion of solid foods later on. According to Professor Andrea Azcarate-Peril, of the University of North Carolina, babies who are breastfed have microbial communities that appear to be more prepared for the introduction of solid foods. Though in its early stages of research, the stability and health of the gut microbiome is supposed to be strongly related to the prevention of allergies. (Telegraph)
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Antibiotics: Antibiotics disrupt the natural balance of microbes in the gut. This then makes it harder for the immune system to distinguish between real attacks and harmless chemicals. (NewsScientist)

THE REACTION

‘Allergie’: Allos- different. Ergia- action

Definitions

T- Cells- A type of white blood cell that plays a vital role in the immune response  (Wikipedia)

Regulatory T cells- A group of lymphocytes that control the immune system and prevent allergic reactions, asthma and autoimmune diseases  (ScienceDaily)

T Helper Cell- Helps other cells in the immune system by recognising foreign antigens to then activate the T cells (MedicineNet)

Antigen- Toxin or foreign substance (Oxford)

Tyramine- A trace amine that comes from the amino acid tyrosine (Wikipedia)

When the immune system encounters an allergen that isn’t harmful for the body a type 1 response occurs. The T cells, found in the thymus gland deem the allergen as harmless. However, in a type 2 response, T cell helpers are put into action to then stimulate the type of antibodies called immunoglobulin E. (MyCityMag)
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An allergic reaction is particularly when a person reacts to the protein in a food and the consumption results in the production of Immunoglobulin antibodies. Other negative reactions to foods are not allergic reactions but rather come under the umbrella of intolerances. As an example, people who react to tyramine in cheeses are not allergic because the reaction triggers the release of histamine and is not an immune response and thus is not an allergic reaction. (ScienceLife)