This form of reaction occurring in sensitized subjects has been known since Arthus1in 1903 reported his experiments. He attributed it to the release of antibodies which seem able to bring about all those degrees of tissue response to insult commonly known as inflammation. Previously Koch in 1891 had described the difference in the reactions to injections of live or killed tuberculosis bacteria in normal and tuberculous animals. He pointed out that in the tuberculous a dose that is not sufficiently large to kill "an animal often produces an extensive necrosis of the skin in the neighborhood of the site of the injection." Pirquet later called this altered response of tissue in the tuberculous "allergy." The mechanism of this spectacular result was not attributed to vasomotor spasm but to contact of antigen and antibody at the site of injection by Valy Menkin,2who, after numerous experiments, wrote: "I