About Allergy

An allergy occurs when the immune system overreacts to an otherwise harmless substance and generates an immune response against the perceived threat. This results in immune memory against that particular substance so that future exposure may cause a more aggressive reaction. The symptoms depend on the nature (inhaled, ingested, contact) and extent of the exposure. Once you develop sensitivity to an allergen, an allergic response is triggered every time you are re-exposed to it.

While you do not inherit a particular allergy directly, you may inherit a tendency to be allergic. It is therefore very common for allergies to run in families. 

Pathophysiology

After the initial exposure the immune system labels the substance as a threat by producing specific IgE antibodies against it. This primes the immune system to react more aggressively to any further exposures. When re-exposure to the allergen occurs, immune cells such as Basophils and Mast cells in particular are activated and release chemicals including histamine, leukotrienes, cytokines and prostaglandins. These chemicals cause allergy symptoms and further activate other immune cells.

The cause for the rising prevalence of allergies is not fully understood but it is known that certain risk factors make allergy more likely. These risk factors include overuse of antibiotics, exposure to air pollution, maternal/passive smoking and climatic factors.

Certainly, environmental exposures are key to understanding the rising prevalence of allergies. The ‘hygiene hypothesis’ was proposed as an explanation whereby the more sterile Western lifestyle was reducing parasitic infections and resulting in less type 1 immune responses. More recently, there is a better insight into the development of allergen tolerance with the microbiome during early life being an essential component. Antibiotic use will disrupt this amongst other environmental factors.

Allergy symptoms are dependent on the nature and degree of exposure to an allergen as well as the degree of immune system sensitisation.

IgE mediated Food Allergy symptoms can include rapid onset of a rash, facial swelling, airway symptoms and sometimes gastrointestinal symptoms such as nausea and vomiting. If these symptoms occur rapidly and quickly worsen then this is usually indicative of a severe reaction known as Anaphylaxis. 

Hay fever (seasonal allergic rhinitis) primarily affects the eyes and nose but it may also  exacerbate asthma and eczema. Symptoms of hay fever typically include sneezing, a runny or congested nose and watery, itchy eyes.

Eczema (atopic dermatitis) is characterised by itchy, red inflamed skin.

Contact Dermatitis is a type of dermatitis that is caused by the skin coming into direct contact with an allergen e.g. nickel. It is characterized by itchy red skin that occurs in the same distribution as where the allergen was in contact with the skin.

Asthma symptoms include cough, wheeze, breathlessness and chest tightness.

Urticarial hives appear like red (or pale) raised patches on the skin. They are typically itchy and can be aggravated by cold, heat, pressure, exercise and stress. Urticaria may occur in response to a wide range of potential triggers.

Allergens can be foods, airborne particles or chemicals.

• The most common inhalant allergens are house dust mite, pollen (grass, tree and weed), animal dander, mould spores and feathers.
• The most common food allergens are egg, milk, nuts, fish and shellfish. In adults, new reactions to food are more likely to be food intolerances rather than an IgE mediated allergic reaction.
• Some insect bites and stings can also cause allergies.

Cigarette smoke and the chlorine found in swimming pools are sometimes considered a trigger of allergic symptoms but these are actually irritants rather than allergens. While irritants can cause symptoms which mimic allergy symptoms they can also aggravate an existing allergic condition such as hay fever, asthma, eczema or urticaria.

There are three reliable methods of allergy testing available: Skin Prick Allergy Testing, RAST IgE specific blood testing and Patch Testing.

Skin Prick Allergy Testing and IgE specific blood testing can be used to detect allergies to a wide range of foods and airborne allergens. Skin prick allergy testing is the primary method used to test for allergies and as such this is the procedure used in Allergy Ireland and all major allergy centres. A pin is touched against the skin to introduce a drop of the suspected allergen. The size of the wheal and surrounding redness on the skin indicates how strongly you are allergic to a particular allergen. Skin prick testing results are available within 15 minutes. It is vital that the results are interpreted by a suitably qualified doctor.

Patch testing is used specifically to identify contact allergic triggers in a skin condition called contact dermatitis. The results must be interpreted by an Allergy Doctor or Dermatologist.

Anaphylaxis is the medical term for a severe and possibly life threatening allergic reaction. It is most often triggered by the ingestion of a specific food or by medical administration of intravenous general anaesthetic or antibiotics. The symptoms typically occur within minutes of exposure and worsen rapidly. Urgent medical treatment is required.