Allergy, or rather the allergic reaction, corresponds to an abnormal, excessive response of the immune system. This reaction is the consequence of a contact with a substance (the allergen) which is foreign to the body but not in and of itself of any danger. Thus, a substance which is entirely inoffensive for most people induces a pathological allergic reaction in sensitised subjects.

• Genes
• Allergens
• The immune system: Lymphocytes, IgE antibodies, Inflammatory mediators, Mast cells
• The target organs: nose, eyes, bronchial tubes, skin, and the gastrointestinal tract