It is a common experience for patients with hay-fever or asthma to find that their symptoms are influenced by heat, cold, mechanical irritation, and change in atmospheric conditions, as well as by exercise, reflexes, or emotional disturbances.
It is very evident that such factors frequently play contributory rôles in the pathogenesis of symptoms due primarily to hypersensitiveness to foods, pollens and other agents. The marked effect of physical conditions is often difficult to account for on this basis, however, for occasionally the nasal and bronchial symptoms seem to be affected solely by a physical agent. This surmise is very difficult to prove true.
A good opportunity for a careful investigation of this relationship is afforded through the study of patients subject to urticaria and allied skin affections who appear to be markedly influenced by the action of physical agents. Here reactions can be more directly and accurately studied than is