Allergies are a physiological error. The body reacts negatively to harmless substances, foods or stimuli. The word "allergy" comes from the Greek allos, meaning "other". It was first used in 1906 to refer to an "altered reaction" in the body's immune system.

An allergic reaction occurs when the body comes in contact with the allergen. This can occur through contact with the skin, inhalation, injection or ingestion of the substance.

Medical treatment includes: avoidance of the allergen, medications to reduce the symptoms of the allergic reaction or injections to desensitize the patient to the allergen. The medical classification of a "true allergy" requires an IgE-mediated response. However, there are also non-IgE-immune sensitivities as well as involvement with IgA, IgG or IgG4.

The use of the word "allergy" has become a generic term used by the public to describe allergies, sensitivities and intolerances. The distinction of a true allergy based on circulating immunoglobulins is generally unknown. Therefore, the term "allergy" is now understood as a term for any negative reaction from exposure to a benign substance.