An allergy can refer to several kinds of immune reactions including Type I hypersensitivity in which a person's body is hypersensitised and develops IgE type antibodies to typical proteins. When a person is hypersensitised, these substances are known as allergens. The word allergy derives from the Greek words allos meaning "other" and ergon meaning "work". Type I hypersensitivity is characterised by excessive activation of mast cells and basophils by immunoglobulin E resulting in a systemic inflammatory response that can result in symptoms as benign as a runny nose, to life-threatening anaphylactic shock and death.

Allergy is a very common disorder and more than 50 million Americans suffer from allergic diseases. Allergies are the 6th leading cause of chronic disease in the United States, costing the health care system $18 billion annually.