What is an allergy?1
An allergy is an excessive sensitivity or reaction of the immune system to substances in our environment that are normally harmless to us (such as pollen, dust mites, or foods). This excessive immune response triggers a series of inflammatory reactions in various organs and mucous membranes of the body, leading to the various symptoms and signs of allergic diseases. Allergic reactions occur in two different types: immediate and delayed hypersensitivity reactions, with immediate reactions (mediated by IgE antibodies) being the most common. When allergic inflammation affects the skin, it results in dermatitis; when it occurs in the conjunctiva of the eye, it results in conjunctivitis; when it affects the nasal mucosa, it results in rhinitis; and when it affects the bronchi, it results in asthma.
What causes allergies?2
Substances that cause allergic diseases are called allergens. They can be defined as harmless substances that trigger hypersensitivity reactions in susceptible individuals.
They cause diseases through contact with the respiratory system (asthma, rhinitis) and/or the conjunctiva of the eye (conjunctivitis). The most common airborne allergens are pollen, mold spores, dust mites, and animal dander.
When consumed, they cause sudden reactions accompanied by digestive, respiratory, and/or skin symptoms. While the most common allergies in children are to proteins in milk and eggs, those in adults are most often to fruits and seafood. Allergies to parasites that contaminate food—such as Anisakis simplex larvae found in fish—are also considered food-related.
When they come into contact with the skin for a short or very long period of time, they cause skin reactions such as eczema or dermatitis. The most common contact allergens are metals (especially nickel) and certain topical medications.
These include medications, latex, and venoms from Hymenoptera (bees and wasps).
What types of allergy tests are there, and how are they performed? 3
In clinical practice, the etiological diagnosis of an allergy is made based on the patient’s medical history, supported by allergy tests. Allergy tests include skin tests and in vitro measurements of specific IgE, which indicate the presence of specific IgE against an aeroallergen.
Skin tests can be performed in several different ways, depending on the allergic condition being treated and the type of allergic reaction suspected (immediate or delayed).
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 applied to the front of the forearm, and the outermost layer of the skin (the epidermis) is gently pricked with a lancet at the site of the drop. A reaction is observed within a few minutes.
Another type of this test is the prick-to-prick test: This test involves pricking a natural substance (such as a piece of fruit) with a lancet and then pricking the skin with the same lancet.
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It is the most important test for diagnosing certain allergic skin conditions, such as allergic contact dermatitis (allergic eczema) or reactions to certain medications.
This substance is usually mixed with petroleum jelly and applied to the skin using special dressings or polyethylene tape. The substance remains on the skin for several days. After 48 hours, the tape is removed and the reaction is observed. After 96 hours, a final measurement is taken without washing the area.
Intradermal test
For certain allergens (such as hymenoptera venom, medications, or individuals with low sensitivity), a skin prick test is not sufficient. In such cases, an intradermal test is recommended because it is more sensitive.
This test involves directly injecting the extract deeper into the second layer of the patient’s skin (the dermis) and taking a reading 15–20 minutes later. It is also useful for evaluating delayed-type hypersensitivity; readings should be taken at 24, 48, and 96 hours.
1. Chivato Pérez, T. “What Is an Allergy? What Does Allergology Study?” In: José Manuel Zubeldia, Mª Luisa Baeza, Ignacio Jáuregui, Carlos J. Senent. BBVA Foundation Book on Allergic Diseases. 1st ed. Bilbao: Editorial Nerea, S.A.; 2012. pp. 21–30.
2. SEAIC: Spanish Society of Allergology and Clinical Immunology (cited on October 10, 2010). URL: 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. The BBVA Foundation’s Book on Allergic Diseases. 2nd ed. Bilbao: Editorial Nerea, S.A.; 2012. pp. 365–370.
