| Image = | Caption = | DiseasesDB = 33481 | ICD10 = | ICD9 = | ICDO = | OMIM = | MedlinePlus = 000812 | eMedicineSubj = med | eMedicineTopic = 1101 | MeshID = D006967 | }} Allergy is a disorder of the immune system that is often called atopy. Allergic reactions occur to environment substances known as allergens; these reactions are acquired, predictable and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody, known as IgE, resulting in an extreme inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees. Mild allergi