DEFINITION OF ALLERGY When the animal body becomes infected with tubercle bacillus, its reactive powers soon become profoundly altered. This deep-seated alteration is in many respects imperfectly understood, but it manifests itself in at least two important ways. In the first place, tubercle bacilli cannot thrive as well in the previously infected body as in the normal one, i. e., an immunity is developed which, while admittedly incomplete, is nevertheless of the greatest importance in restraining the growth of, and preventing further invasions by, the bacillus. In the second place, this immune body is abnormally susceptible to protein derived from the body of the tubercle bacillus. Locally, amounts of tuberculoprotein that are rather harmless to the normal body produce necrosis of tissue and intense inflammation in the infected one. Intravenously, amounts that will be ignored by the normal body promptly produce fever, prostration and even death in the infected one.