1. (Science: immunology) a state of hypersensitivity induced by exposure to a particular antigen (allergen) resulting in harmful immunologic reactions on subsequent exposures, the term is usually used to refer to hypersensitivity to an environmental antigen (atopic allergy or contact dermatitis) or to drug allergy.
The original meaning, now obsolete, included all states of altered immunologic reactivity, immunity as well as hypersensitivity. Gell and Coombs used the term allergic reaction to mean any harmful immunologic reaction causing tissue injury.
2. (Science: study) The medical specialty dealing with diagnosis and treatment of allergic disorders. Hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity.An allergy is an antigen that causes an allergic reaction as the result of foreign agents present inside the body which antibodies attempt to break down and destroy.