MSH: Altered reactivity to an antigen, which can result in pathologic reactions upon subsequent exposure to that particular antigen.,CSP: state of previously immunized body in which tissue damage results from the immune response to a further dose of antigen.,HL7V3.0: Hypersensitivity to an agent caused by an immunologic response to an initial exposure ,NCI: An exaggerated response by the immune system to a drug or other substance.,GO: An inflammatory response to an exogenous environmental antigen or an endogenous antigen initiated by the adaptive immune system. [GOC:jal, ISBN:0781735149],MEDLINEPLUS: An allergy is a reaction of your immune system to something that does not bother most other people. People who have allergies often are sensitive to more than one thing. Substances that often cause reactions are Pollen Dust mites Mold spores Pet dander <a href='http://www.nlm.nih.gov/medlineplus/food