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What is an Allergy?

An allergy is the response of the body's immune system to normally harmless substances, such as pollens, foods, and house dust mite. Whilst in most people these substances (allergens) pose no problem, in allergic individuals their immune system identifies them as a’ threat’ and produces an inappropriate response.


Allergies are classified into IgE mediated and non-IgE mediated allergies. In IgE mediated allergies the immune system produces exaggerated amounts of a distinct class of antibodies known as IgE antibodies that are, specific for the particular offending allergens. These IgE antibodies bind to the surface of cells in the body called mast cells which become ‘IgE-sensitised’ such that these cells can then identify particular allergens the next time they come in contact with the body. This process is called sensitisation, and at this stage there are no physical symptoms of an allergy.

Mast cells are present in tissues that are in contact with the external environment, including the skin, nose, eyes, mouth, throat, stomach and gut. The next time that the same allergen is encountered the mast cells identify it as an intruder and produce histamine and other chemicals. It is the release of these chemicals from mast cells and their effects on the body that result in allergic symptoms.

IgE-mediated allergy may cause a wide spectrum of symptoms depending on the allergen and the site of the body affected. In the nose histamine release results in symptoms of runny nose,  itchy nose sneezing (rhinitis) that are commonly associated with itchy red eyes (conjunctivitis). In the skin symptoms include redness and nettle rash (hives, wheals). In the breathing tubes allergies cause wheezing, cough and shortage of breath (asthma), whereas in the gut symptoms such as abdominal discomfort (‘tummy ache’), nausea, vomiting and diarrhoea may occur. Severe allergies may result in throat swelling, severe asthma and a drop in blood pressure. Severe allergic reactions are also known as anaphylaxis, and can be life-threatening.

The immune system may also respond to allergens without the production of the IgE antibody. The mechanisms of these so called non-IgE mediated allergies are far less well understood and are likely to involve multiple cells that react inappropriately to the presence of an allergen. Whereas symptoms of IgE mediated allergies occur rapidly and soon after exposure to the allergen, this may not be the case with non-IgE mediated allergies where symptoms may appear much later. In these cases it can be much harder to determine whether the problem is allergic in nature and if so which particular allergen is causing the problem

Allergy is widespread and affects approximately one in four of the population in the UK at some time in their lives. Each year the numbers are increasing with as many as half of all those affected being children.


Last reviewed: October 2013                   Next review date: October 2016
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Last updated: March 2012

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