Definitions of Hypersensitivity, Allergy, and Atopy
Posted Oct 24 2009 10:03pm
There are three terms have been used, often interchangeably, but this causes confusion. Below are definitions from theEuropean Academy of Allergy, Asthma and Immunology (EAACI) Position Statement on Nomenclature. I try to stick with these when I write, lecture, or speak with patients. I think definitions must be agreed upon, because they are the building blocks we use to communicate ideas. Their meaning can change, but we should still agree on the meaning, otherwise we will automatically be miscommunicating.
1. Hypersensitivity causes objectively reproducible symptoms or signs, initiated by exposure to a defined stimulus at a dose tolerated by normal subjects. There are allergic and non-allergic subtypes (e.g. aspirin reactions). This definition does not accommodate classical responses to infection, autoimmunity, or toxic reactions.
2. Allergy is a hypersensitivity reaction initiated by immunologic mechanisms. Allergy can be antibody- or cell-mediated. In most patients, the antibody typically responsible for an allergic reaction belongs to the IgE isotype and these patients may be said to suffer from IgE-mediated allergy. In a minority of patients, the reactions may be either IgG antibody-mediated (e.g. hypersensitivity pneumonitis), or T-cell mediated (e.g. contact dermatitis), or other cells (lymphocytes cause many disorders: e.g. some eczema, celiac disease).
3. Atopy is a personal or familial tendency to produce IgE antibodies in response to low doses of allergens, usually proteins, and to develop typical symptoms such as asthma, rhinoconjunctivitis, or eczema/dermatitis. It is not always possible to demonstrate a person is ‘‘IgE sensitized’’ or ‘‘skin prick test positive,’’ because of their age (infants), or the discomfort of in depth testing, or the wrong allergens were tested, or the skin prick tests are simply not good enough or numerous enough to detect the IgE antibodies (some dust and mold allergies).