Allergy is a disease of the immune system. This enables our body to defend itself against external attacks (bacteria, viruses, parasites…). The immune defence is composed of 2 phases: the recognition of the attacker (specific response), and then the use of different mechanisms to neutralise the attacker (inflammation). The specific response is the recognition of a protein foreign to the body (called antigen) by antibodies or specific cells. In allergy, the antigen (called allergen in this case) is recognised by a specific class of antibodies called IgE. These IgE trigger a specific inflammatory response: allergic inflammation. This has characteristics that allow it to be distinguished from the other types of inflammation.
The allergic response requires an initial exposure to the allergen, which activates the production of IgE specific to that allergen (sensitisation phase). Only after further exposure can the allergic reaction take place. Exposure to the allergen, even in tiny amounts, is essential for an allergic reaction to occur. There are several diagnostic methods available to the doctor to detect an allergy: Skin tests consist of bringing the allergen into contact with the skin. If the patient is sensitised, a skin reaction will occur at the site where the allergen was deposited.
There are mainly 3 types of skin tests:
– Prick tests, where the allergen is introduced into the top layer of skin using a lancet. The result is read after 15 or 20 minutes.
– Intradermal tests, where a small amount of allergen is injected into the skin using a syringe. The result is read after 15 to 30 minutes.
– Patch tests, where the allergen is applied to the skin and covered with plastic. The result is read after 2 to 4 days.
A blood analysis of the patient makes it possible to measure the allergy antibodies (IgE) specific to the allergen. Other, more sophisticated and less frequently performed tests are also available.