=Anaphylaxis.= The conception _anaphylaxis_ or _lack of protection_ begins with the discovery that a harmless protein injected into a dog will so sensitize him that, after ten days or so, another injection of the same protein will kill him. The point is that the change has occurred in the animal, not in the protein injected. The protein is the same as before and can be injected once into any number of dogs without harm. In this way we explain the cases in which drugs and foods that are harmless to most people may be virulent poisons to those who happen to have been sensitized by a former overdose. The widespread use of antitoxin in diphtheria gave abundant opportunity to study the phenomena of sensitizing a human being with one dose and killing him with another dose of the same thing. The symptoms of anaphylaxis first observed were urticaria, arthritis, and dyspnoea. Then Bruck showed that what we used to call _idiosyncrasy_ to drugs and foods that are harmless to most people is really an anaphylaxis, attributable to a former overdose of the same thing. Next, it was learned that anaphylaxis may persist through life and be transmitted to the offspring of rabbits and guinea-pigs, illustrating the cases in human families where sensitiveness to a certain food or drug runs down through several generations. Then the dermatologist brought in a list of skin eruptions, urticaria in the lead, as examples of anaphylaxis to certain foods or to poisons generated within the body, especially in the intestines. Then asthma was included among the anaphylactic reactions and, finally, Wolff-Eisner pointed out that the lesion of hay fever is an anaphylaxis. I may add here that this view of hay fever confirms my observation that the lesion is not a catarrhal inflammation but an urticaria.