In the pathogenesis of bronchial asthma mediated by specific hypersensitiveness, hereditary predisposition, as well as the immunologic mechanism, frequently plays an important role. This is especially common among atopic patients. The immunologic mechanism is regarded as involving an antigen-antibody reaction on or within certain cells or tissues designated as shock organs. These organs are likewise usually subject to hereditary influences. The particular site of the allergic reaction, whether the epithelium, the smooth muscle or the endothelium of blood vessels, has been subject to a great deal of controversy. It is the purpose of this paper to review this phase of the problem and to indicate that the vascular apparatus plays a basic role in the production of altered tissue reactivity in man. A number of arguments support this hypothesis. According to Lewis1and Dale2the antigen-antibody interaction is supposed to result in the liberation of a histamine-like substance