About 100 years ago, a young paediatrician understood that the function of the immune system should be rationalized not in terms of exemption of disease but in terms of change of reactivity. He coined a new word to represent such an idea: 'allergy': the first contact of the immune system with an antigen changes the reactivity of the individual; on the second and subsequent contacts, this change (or allergy) can induce a spectrum of responses from protective (literally, immune) to hypersensitivity ones. The idea was at first hardly understood by the scientific community because it undermined the essentially protective nature of the immune response as it was defined. Nevertheless, in the next years, the growing clinical evidence led to the acceptance of this new point of view, but not of the new word, at least not unconditionally. The original significance of the neologism 'allergy' became perverted and limited to describe hypersensitivity conditions. Perhaps because of the corruption of the term, today 'allergy' does not have a well-delimited significance among health professionals. Furthermore, the word has long ago escaped from physicians and gone to the streets, where it is popularly used also as synonymous with antipathy and rejection. This vulgarization of the term 'allergy' has significantly increased its imprecision.