IMMUNITY AND HYPERSUSCEPTIBILITY 233 



nearly always an immunity remains long after such an excess 

 has disappeared. It would seem that the ability of the cells of 

 the body to respond promptly to invasion is often developed by 

 ejxperience with one such invasion, and that this ability may re- 

 main for a long time as a factor in immunity. 



Hypersusceptibility or Anaphylaxis. — If a guinea-pig be in- 

 jected with a small amount of a protein, such as egg-albumen or 

 blood serum 6i the horse, and then after an interval of ten to 

 iwenty days be injected with a second dose of the same protein 

 of good size (0.5 to 5 grams), the animal usually develops symp- 

 toms of nervous intoxication and often dies within a half hour. In- 

 asmuch as normal guinea-pigs withstand enormous doses of such 

 protein substances, it is evident that the first injection has brought 

 about some change in the animal, an altered reactivity, which 

 results in the intoxication after the second dose. That this phe- 

 nomenon of hypersusceptibihty or anaphylaxis { = against pro- 

 tection) bears a definite relation to immunity may be illustrated 

 by an experiment in which ' typhoid baciUi are substituted for 

 the soluble protein. If a guinea-pig be immunized by repeated 

 doses of the killed micro-organisms he is able to resist inoculation 

 with an ordinarily fatal dose of the living germs, which are quickly 

 killed and dissolved by the specific bacteriolysins in the body 

 fluids. However, if such an immune guinea-pig be injected with 

 a proper dose of dead organisms, which would not kill a normal 

 animal, he may quickly succumb. The abiHty of the body fluids 

 of the immune animal to disintegrate the bacterial cells rapidly 

 would seem to be the factor upon which depends not only its 

 immunity to the small dose of living germs, but also its exagger- 

 ated ' sensitiveness to dead germ substance. The products of the 

 rapid parenteral digestion of the foreign protein would seem to be 

 the cause of the symptoms of intoxication. The essential unity 

 of the substances upon which immunity and anaphylaxis depend 

 has been emphasized by Von Pirquet^ and his co-workers.' 



' Von Pirquet: Allergy. Archives of Internal Medicine, 1911, Vol. VII, pp. 259-288; 

 PP- 383-436. 



