WHAT IS ALLERGY?
Allergy can be defined as an inappropriate or over-reaction of the immune system that has the potential to cause symptoms of allergic diseases such as hay fever, asthma, eczema food allergy or anaphylaxis.
Allergy is generally used to refer to symptoms associated by type 1 hypersensitivity triggered by allergic antibodies (termed IgE). Up to 50% of the population make such IgE antibodies that recognise common proteins in the environment including those in pollens, animal dander, dust mites, mould spores and foods termed allergens.
Antibodies are part of the immune system and recognise foreign proteins that the individual has previously been exposed to, acting as a memory to trigger defence mechanisms: for example vaccination works in part by triggering antibody production against harmful viruses or bacteria that activate white blood cells to kill these organisms if they enter the body. Each antibody interacts with one particular protein structure.
IgE antibodies bind to receptors or special docking sites on immune cells such as mast cells or basophils and when triggered by the allergen they recognise, cause those cells to rapidly release stored chemicals including histamine. This reaction is very rapid occurring within minutes of exposure. Histamine causes blood vessels to expand to increase blood flow (hence the site of reaction looks red), causes leaking of fluid from the blood vessels into the tissues resulting in swelling or oedema, and activates small nerve fibres leading to itching, sneezing, wheezing depending on where the reaction occurs.
Allergic sensitisation can be confirmed by demonstrating the presence of IgE antibodies against a particular allergen (for example grass pollen extract) by skin prick tests which reproduce an allergic reaction in the skin in a safe and controlled manner or by measuring concentrations of allergen-reactive IgE in the blood using a RAST test.
IT IS IMPORTANT TO NOTE THAT THE DETECTION OF IgE ANTIBODIES TO AN ALLERGEN DOES NOT NECESSARILY MEAN THAT ALLERGEN IS CAUSING ANY SYMPTOMS: THESE TESTS HELP WITH DIAGNOSIS WHICH MUST INCLUDE A CAREFUL HISTORY.
However the absence of IgE to a particular allergen (for example cat dander) generally means that allergen will not cause allergic symptoms. It is also of note that some symptoms do not have an allergic basis, that is, they occur in the absence of IgE sensitisation.
Why is confirmation of allergic sensitisation useful?
It can help in guiding avoidance strategies as in food allergy, whether there is risk of anaphylaxis as in food allergy or drug allergies, and may guide allergen specific therapies such as allergen immunotherapy or anti-IgE treatment.
Some specific types of allergy result from distinct mechanisms: contact dermatitis or skin allergy to metals for example does not involve IgE. Sometimes allergic type symptoms do not have an external allergen as their trigger: examples are non-allergic asthma and rhinitis or many cases of chronic skin weals .
Further information on allergies is available on the ALLERGY UK website.