INTRODUCTION 25 



passage through the cow. The anthrax bacillus is con- 

 verted into a vaccine by growth at high temperature. 

 The typhoid bacillus is killed by heat. In all these instances 

 the protein is so little changed in being converted from an 

 active, infecting agent into a vaccine that it still sensitizes 

 the animal to itself in the unmodified state. It seems 

 reasonable to conclude that the protein retains its capa- 

 bility of sensitizing so long as there is no radical alteration 

 in its chemical structure. The results secured by vaccina- 

 tion with killed typhoid bacilli prove that in this instance 

 at least a vaccine is not necessarily a living organism. The 

 possibility of obtaining vaccines from the split products of 

 pathogenic proteins has led to some of the investigations 

 detailed in this volume, and while we do not claim success 

 in this particular, we think that continued efforts in this 

 direction are justified. 



12. Protein sensitization and bacterial immunity, appar- 

 ently antipodal, are in reality identical. This statement first 

 made by us in 1907 has since met with wide acceptance, 

 and will be discussed in detail in later chapters. 



13. Protein sensitization consists in developing in the 

 animal body a specific proteolytic ferment which digests the 

 same protein on reinjection. Protein sensitizers may be 

 living or dead, particulate or in solution. Soluble proteins 

 sensitize more readily and more fully than those not in 

 solution. The development of the specific proteolytic 

 ferment in sensitization is due to the action of the foreign 

 protein upon the body cells. There is developed in certain 

 body cells a new function, that of elaborating this new 

 ferment. In order for a given body cell to be thus influenced 

 by a foreign protein, the latter must come in contact with 

 the former. Cell permeation by the foreign protein is 

 probably essential to the perfect elaboration of this process. 



14. When a foreign protein is introduced into the blood of 

 an animal it soon leaves the circulating fluid and is distributed 

 throughout the tissues. The truth of this has been demon- 

 strated by the researches of independent investigators, and 

 will be detailed later. This is true of both particulate and 



