I. Introduction In vitro and in vivo models of inflammation based on mast cell triggering by antigen have played an important role in delineating many of the mechanisms responsible for producing physiological and pathological changes characteristic of asthma. These models, particularly those using in vivo challenge of allergic human volunteers, have advantages of both relevance and ability to control the timing, duration, and intensity of exposure to a specific triggering agent Initial observations made in the nineteenth century demonstrated that antigen inhalation could produce symptoms several hours later (l ). However, systematic investigations of antigen-induced late airway reactions were first made by Herxheimer in the middle of this century (2), and then later by Booij-Noord et al. (3) and Pepys (4). (The early and late asthmatic responses to allergen are the topic of Chapter 34.) Work performed subsequently by a large number of investigators studying the physiological and pharmacological characteristics of airway reactions induced by antigen, particularly late phase reactions ( 1 ,5-8) has provided the foundation for many of the more invasive studies being performed today. These invasive studies were made possible by the demonstration that asthmatics could undergo fiberoptic bronchoscopy, bronchoalveolar lavage (BAL), and endobronchial for-
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