Allergy is a medical term which is used to describe a reaction within the body to a substance (which is normally harmless itself), which results in an immune response (including Type I hypersensitivity in which a person's body is hypersensitised and develops IgE type antibodies to typical proteins) and a reaction that causes symptoms and disease in a predisposed person. That can be a source of mild or extreme inconvenience, and even a great deal of misery.

Allergens are the substances causing the reaction in a hypersensitised person. The word allergy derives from the Greek words allos meaning "other" and ergon meaning "work".

Type I hypersensitivity is characterised by excessive activation of mast cells and basophils by immunoglobulin E resulting in a systemic inflammatory response that can result in symptoms as benign as a runny nose, skin rash and itchy eyes, to life-threatening anaphylactic shock and sometimes fatality. It effects the sense of sight, smell, tastes and touch and is the result of the body's immune system overreacting to normally harmless substances.

What causes an Allergy?

Allergic reactions are caused by substances in the environment known as allergens. Almost anything can be an allergen different for people. Allergens contain protein, which is often regarded as a constituent of the food we we eat.
In fact it is an organic compound, containing hydrogen, oxygen and nitrogen, which form an important part of living organisms.

The most common allergens are:

* pollen from trees and grasses;
* house dust mite, moulds;
* pets such as cats and dogs;
* insects like wasps and bees;
* industrial and household chemicals;
* medicines, and
* foods such (as milk and eggs).

Less common allergens include:

* nuts,
* fruit and
* latex.

There are some non-protein allergens which include drugs such as penicillin. For these to cause an allergic response they need to be bound to a protein once they are in the body.

An allergic person's immune system believes allergens to be damaging and so produces a special type of antibody (IgE) to attack the invading material. This leads other blood cells to release further chemicals (including histamine) which together cause the symptoms of an allergic reaction.

The most common symptoms are:
sneezing , runny nose, itchy eyes and ears, severe wheezing, coughing shortness of breath, sinus problems, a sore palate and nettle-like rash.

It should be understood that all the symptoms mentioned can be caused by factors other than allergy. Indeed some of the conditions are diseases in themselves.

When asthma, eczema, headaches, lethargy, loss of concentration and sensitivity to everyday foods such as cheese, fish and fruit are taken into account the full scale of allergy be appreciated.