What is an 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, called 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 or 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.

The detection of IgE antibodoies in response to an allergen does not necessarily mean that allergen is causing any symptoms. These tests help with diagnosis, which must include a careful history.

On the other hand, 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.

Bee and wasp sting allergies

Severe allergic reactions can occur to bee or wasp stings. Patients who have had a rash at sites other than the sting, faintness or difficulty breathing should be assessed by an allergist as immunotherapy is very effective. They should be prescribed injectable epinephrine (Epipen or similar) and shown how to use it.

Large swellings at the site of a sting can be treated with antihistamine but do not need epinephrine or immunotherapy.

Drug allergies

Reactions to medicines are common but IgE sensitisation is relatively rare. Careful history is needed and if the drug is essential for care a supervised challenge test may be needed.

Food allergies

Food allergy is relatively common in young children, with milk or egg allergy often associated with eczema. In adults type 1 food allergy is most often to nuts or peanuts or fish.

Severe type 1 reactions can occur in patients with IgE to foods: particularly to nuts. These can cause swelling of the mouth and throat, generalised itching and swelling (oedema), difficulty breathing (due to throat swelling or asthma) and a fall in blood pressure leading to collapse. Such reactions must be investigated thoroughly and patients must be advised on appropriate food avoidance and carrying an injectable syringe of epinephrine (Epipen, ALK or similar) which reversed some of the effects of anaphylaxis. Such patients should have clear management plan, may carry a warning bracelet or necklace (such as Medic-Alert) and can gain useful information from the Anaphylaxis Society. Food anaphylaxis can occur to many foods types (including sesame seed, specific mils, fruits etc) and requires careful history and investigation with skin or blood tests.

Other food reactions may be less severe. These include cross reactions between allergen in foods and other allergens to which the patient is sensitised. Examples are itching in the mouth due on eating apples, pears or other fruits due to birch pollen allergy, or reactions to kiwi, avocado or banana due to latex allergy. These symptoms can generally be managed by avoidance and anti-histamines.

Food intolerance

There is a wide variety of symptoms associated with foods that do not result from IgE to the food. These tend to be more variable (they don’t always occur when the food is eaten), more dose related (often only occurring when large quantities are ingested) and generally cause gut symptoms (typically bloating or diarrhoea). These symptoms should be investigated, malabsorption ruled out and often avoidance is helpful.

It is important to make sure that avoidance diets remain balanced and dietetic help is invaluable.

Latex allergy

Latex allergy is relatively rare and usually occurs in heath care workers using rubber gloves, or patients needing repeated operation or urinary catheters (such as those with Spina bifida). It can result in anaphylaxis and accurate diagnosis and careful avoidance measures are essential.

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 ALLERGY UK.