Allergy is a state of hypersensitivity induced by exposure to a particular antigen (allergen) resulting in a harmful immunologic reaction. An antibody is a protein molecule made by the body to help combat disease-causing viruses and bacteria. The antibody binds to its antigen which is usually a chemical located on the bacterium or virus, so the end result is that the antibody binds to the invader. The bound antibodies are rather like little fingers pointing to the invader, their presence arouses the bodies defence (immune) cells and they attack the invader.

Unfortunately in allergy the body makes IgE antibodies (which are usually found on the surface of specific immune cells known as mast cells, that are to be found in tissues throughout the body). If the IgE molecules on the surface of the mast cell bind to their specific antigen they stimulate the mast cell to release several chemical messengers. The usual purpose of these chemicals is to organise a more effective immune response, but in sufficient quantities they can produce the serious symptoms of allergy.

In this type of allergy, reactions can occur very quickly, with symptoms like asthma, hayfever, eczema and worst of all anaphylaxis, and can usually be diagnosed by skin prick test. Skin prick tests should only be done if there is resuscitation equipment in the same room or by RAST - a blood test that is done under a microscope, with no danger whatsoever to the patient. The allergic patient will almost always have a raised IgE level. Things most likely to cause allergy are insect stings, some ingested chemicals, latex rubber and certain foods like strawberries, shellfish, fish, nuts and eggs. Other foods can and do cause anaphylaxis, but not so frequently.