The aim of the present article is to report the construction of a medical knowledge organization system (MKOS) for the Allergy Unit of the Montpellier University Hospital in France, according to a utility-based contextual approach. The inputs from the analysis of information practices of allergy employees and of a corpus of documents used in the Allergy Unit, as well as validation meetings involving allergy professionals, patients, and KO specialists led to the creation of a multidimensional, multiviewpoint and user-friendly ontology. The ontology is potentially useful to support activities of allergy actors, even though its operational, conceptual, terminological and technical aspects still need to be improved. Such a result suggests that the utility of MKOSs is the matter of the content, intentionality and operationality, and should be constructed by designers through contextual interactions with potential users. The idea has been put into a methodological framework, and a Utility in Context Model has been proposed.