What is an Allergy?
An allergy is the response of the body's immune system to a normally harmless substance, such as pollen, food, or house dust mite. The body has an automatic reaction to what it sees as a threat, and while in most people these substances pose no problem, in those with allergies the immune system identifies them as a threat and produces an inappropriate response to them. Allergies are classified into IgE mediated and non-IgE mediated allergies.
Allergies first start when cells in the immune system wrongly identify an everyday, normally harmless, substance as an attacker. In IgE mediated allergies the immune system then begins to produce begins to produce a class of antibodies known as IgE, specific for that particular allergen, which will later alert the fighting cells (mast cells and basophils) within the immune system every time that this substance is encountered. The mast cells bind with the IgE antibodies so that they can identify the allergen next time it comes into contact with the body. This is called sensitisation, and at this stage there are no physical symptoms of an allergy.
Mast cells are present in all the tissue that is in contact with the external environment, such as 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 the histamine and other chemicals and their effect on the body that cause allergic symptoms.
An allergy can therefore cause anything from a runny nose, or itchy eyes and mouth, to skin rash and gastrointestinal symptoms such as abdominal discomfort (‘tummy ache’) and vomiting. Severe allergies can cause breathing problems or a drop in blood pressure. Severe allergic reactions are also known as anaphylaxis, and can be life-threatening.
In prolonged exposure to allergens the immune system also employs additional fighting cells to attack the invading substance. These release chemical substances that cause further discomfort to allergy sufferers and increase the severity of their symptoms.
However, the immune system can still respond to allergens without the production of the IgE antibody. In non-IgE mediated allergies multiple cells may inappropriately react to the presence of an allergen, and can cause many of the same symptoms as IgE mediated allergies.
Symptoms of IgE mediated allergies occur rapidly and soon after exposure to the allergen, whereas in non-IgE mediated allergies symptoms tend to appear much later after contact with the allergen. In these cases it can be much harder to identify which 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 by 5%, with as many as half of all those affected being children.
Video on the subject
Last updated: March 2012