Allergy

An overreaction of the immune system caused by a particular substance

The symptoms of allergic reactions often vary; extreme cases can lead to life-threatening anaphylactic shock.

In most cases, allergies are caused by a protein that originates outside the body (allergen). The allergen is perceived by the body as a potentially dangerous, alien component, and an immune response is recruited. This leads to a massive release of pharmacologically active substances, predominantly histamines. Such mis-targeted immune responses are called "true" allergies, sometimes confused with pseudoallergies.

Every "true" allergy is preceded by a phase of sensitisation. First, the immune system adapts to the "alien" substance and produces antibodies. A reaction is triggered only by later contact with the allergen. The first cases of allergies to kiwi fruits did not occur until years after their introduction to the market.

A food allergy is an allergic reaction caused by food constituents or their degradation products, additives, residues, or associated microbial compounds (e.g. fungal or microbial toxins). The uptake of the particular allergen most often occurs via mucous membranes in the mouth and intestine, but can also occur through the skin or by inhalation (e.g. flour dust). Figures on the prevalence of food allergics vary; for adults it is estimated at one to five per cent, while for children under six years estimates range from three to ten percent. In Europe, the most common food allergies are to peanuts, soy, cows' milk, hens' eggs, fish, nuts, crustaceans, and celery. Food allergies are less common than other allergies such as to pollen.

Cross sensitivities can occur when an allergen can be found in more than one source. For example, people allergic to birch pollen often also have allergies to apples or nuts.