These substances are called allergens, and the most common are proteins in pollen from trees and grasses, food, fur, dust mites or mould, in addition to metals and substances used in cosmetics and pharmaceuticals.
An allergic reaction is an overreaction of the immune system to an allergen. Allergy development takes place in two stages, with an initial sensitisation or induction phase followed by an effector phase (see figure below). Clinical symptoms of allergy only appear in the effector phase. Some people do not progress from the induction phase and will therefore not present clinical symptoms of allergy.
The figure shows the phases of allergy development
Food allergy is a significant public health problem and is reported to affect 3-4% of adults and 6-8% of children in Europe, with an increasing incidence among children. Changes in eating habits, with new imported products, more processed foods and increased travel can also have contributed to food reactions becoming more common. Food allergy occurs most often in those who have atopic diseases, such as atopic dermatitis, asthma and hay fever.
Allergy is defined as an immunological response and is limited to food reactions that can be confirmed by immunological methods such as the detection of IgE antibodies in serum. Food allergies are triggered by food proteins and usually cause quick reactions. These reactions occur because the natural tolerance is broken. Symptoms are commonly seen in other organs than the stomach and intestines, such as the skin, the respiratory system and the cardiovascular system. Hypersensitivity reactions generally cause intestinal symptoms. Common food allergens include legumes such as peanuts and nuts, flour, fish and shellfish. Cow's milk, wheat and eggs often trigger allergies among children, but they usually grow out of these allergies by school age.
The incidence of severe reactions to food in Norway is unknown. In 2000, the National Register of Severe Allergic Reactions to Food was established to increase knowledge, and doctors have been encouraged to report severe patient reactions. As from January 1st 2018 the Register is no longer in operation.
Respiratory allergies are caused by proteins in the air that are inhaled and trigger airway inflammation. They may be due to specific allergic reactions, or more general reactions to irritants such as smoke and fumes in the indoor and outdoor environment that can aggravate allergy symptoms.
There are many sources of respiratory allergies; house dust contains various components such as dust mites, pet allergens, pollen and particulates that can trigger asthma and allergies. If humidity is too high, mould growth can occur, and the amount of mould spores containing allergens will increase. House dust mites thrive wherever it is hot and humid, especially in mattresses and bedding. Mite faeces are the main cause of allergies. Pet allergens usually come from proteins from animal skin, fur, urine and saliva. The allergens often sit on small particles that can stay aloft for a long time and thus spread over large areas.
The increase in allergic diseases may be related to climate change. A warmer climate leads to a longer pollen season and therefore an increased incidence of respiratory allergies. Pollen can cause cross-allergies because there are similar proteins in pollen and plant foods such as fruits, vegetables and nuts, which will also increase allergic reactions to food. Cross allergies, however, cause milder symptoms than primary allergies.
There are several different types of skin allergies such as allergic contact dermatitis, urticaria (hives) and photoallergy. Clinical symptoms for allergic contact dermatitis often occur 1-2 days after exposure to an allergen. Photoallergy symptoms may occur somewhat earlier than allergic contact dermatitis. Urticaria often occurs within minutes after exposure.
The most common causes of skin allergies are exposure to metals and chemicals that can penetrate the skin, but proteins may also cause skin allergy. These substances are present in consumer products such as cosmetics and body care products, hair dyes, detergents, textiles and jewellery. The most common contact allergens are nickel, fragrances and thimerosal (preservative). Approximately 20% of the population has a contact allergy.
Certain occupational groups are particularly vulnerable to developing skin allergies. These are professions with frequent hand washing and work with irritant and sensitising agents (e.g. rubber chemicals, fragrances, preservatives, dyes, metals).
Atopic dermatitis (sometimes called infantile eczema) is a chronic disease with an itchy rash that often appears in early childhood. About 70% of children with atopic eczema grow out of it before adolescence. The trigger factors are unclear, but children with allergic and asthmatic parents have an increased risk of developing atopic dermatitis. A combination of genes and environment seems to be important.
Undesirable effects from the use of cosmetic products often occur. These include soap, shampoo, moisturisers, deodorants, shaving products and make-up. Undesirable effects include allergies, rashes, blisters, burning or other symptoms.
The products that cause most problems are fragrances, preservatives and hair dyes. Preservatives and fragrances can cause adverse effects, particularly in the form of rashes and other skin reactions.