What is allergy?1
Allergy is a hypersensitivity or overreaction of the immune system to elements in our environment that are normally harmless (e.g. pollens, dust mites or food). This immune overreaction triggers inflammatory responses in various organs and mucous membranes, leading to different symptoms and signs of allergic diseases. Allergic reactions occur in two different types as immediate and delayed hypersensitivity reactions, and immediate reactions (mediated by IgE antibodies) are the most common. Allergic inflammation is referred to as dermatitis when it affects the skin, conjunctivitis when it occurs in the conjunctiva of the eye, rhinitis when it affects the nasal mucosa or asthma when it involves the bronchi.
What causes allergy?2
The agents that cause allergic diseases are called allergens. These can be defined as harmless substances that cause hypersensitivity reactions in susceptible individuals.
They cause diseases through inhalation, affecting the respiratory tract (asthma, rhinitis) and/or the conjunctiva of the eye (conjunctivitis). The most common aeroallergens are pollens, fungal spores, dust mites and animal epithelium.
Once consumed, they cause immediate reactions with digestive, respiratory and/or skin symptoms. The most common allergies in children are to proteins in milk and eggs, while in adults they are mostly fruits and seafood. Allergies to parasites that contaminating food, such as Anisakis simplex larvae in fish, are also considered foodborne.
They produce cutaneous reactions upon contact with the skin for a more or less prolonged period of time, manifesting as eczema or dermatitis. The most common contact allergens are metals (especially nickel) and some topical medications.
They are medicines, latex, Hymenoptera venom (bees and wasps).
What types of allergy tests are there and how are they performed? 3
In clinical practice, the etiological diagnosis of allergy is made based on the clinical history complemented by allergy testing. Allergy testing includes skin tests and in vitro measurements of specific IgE, which indicate sensitization, i.e., the presence of specific IgE against aeroallergens.
Several types of skin test can be performed, depending on the allergic disease being treated and the type of allergic reaction (immediate or delayed) suspected.
Prick-test
It is the most common and most frequently performed allergy test. It is used to detect immunoglobulin E (IgE)-mediated allergies (also known as immediate hypersensitivity reactions).
To perform this test, a drop of the allergen extract to be tested is placed on the forearm and the outermost layer of the skin (epidermis) is gently punctured with a lancet. The response is obtained within a few minutes.
Another variation of this test is the prick to prick test: This test consists of dipping a lancet into a natural substance (e.g. a fruit) and then puncturing the skin with the same lancet.
Patch test
This is the main test for investigating certain allergic skin diseases, such as allergic contact dermatitis (allergic eczema) or certain drug reactions.
The substance is usually mixed with petroleum jelly and applied to the skin using special dressings or polyethylene tapes. The substance remains on the skin for several days. After 48 hours, the patches are removed and the reaction is observed. After 96 hours, a final measurement is taken without washing the area.
Intradermal test
For some allergens (hymenoptera venom, drugs or people with weak sensitivity), the prick test is not sufficient. In these cases, intradermal tests are recommended as they are more sensitive.
This test involves directly injecting the extract deeper into the second layer of the patient's skin (dermis) and taking measurements 15-20 minutes later. It is also useful for studying delayed-type hypersensitivity; measurements should be taken at 24, 48 and 96 hours.
1. Chivato Pérez T. ¿Qué es la alergia? ¿Qué estudia la Alergología? En: José Manuel Zubeldia, Mª Luisa Baeza, Ignacio Jáuregui, Carlos J. Senent. BBVA Foundation book of allergic diseases. 1st edition Bilbao: Editorial Nerea, S.A.; 2012. pp. 21-30.
2. SEAIC: Sociedad Española de Alergología e Inmunología Clínica (cited on October 10, 2010). Available at: https://www.seaic.org/pacientes/conozca-sus-causas
3. Parra Arrondo A. Skin tests. In: José Manuel Zubeldia, Mª Luisa Baeza, Ignacio Jáuregui, Carlos J. Senent. Libro de las enfermedades alergénicas de la fundación BBVA. 2nd edition. Bilbao: Editorial Nerea, S.A.; 2012. pp. 365-370.
