Cutaneous reactions in hypersensitive individuals are of two quite distinct types. The reactions frequently observed in patients with hay-fever or bronchial asthma and those obtained after sensitization by foreign serum consist essentially of an urticaria-like lesion with a central wheal and a surrounding zone of erythema. Such reactions appear a few minutes after application of the test substance, usually reach their full development in half an hour or less, and fade out completely in one or two hours. The skin, at the site of the test then appears normal. There is no visible evidence of cell destruction. It appears to be essentially a vascular phenomenon with localized edema. Such reactions may often be obtained in hypersensitive individuals with extracts of pollens, animal hair, dandruff or feathers, with foreign serum, with many food proteins and occasionally with bacterial or other proteins.
These immediate urticaria-like reactions have little in common with the