IMMUNITY IN TUMOR TRANSPLANTATION 401 



which becomes manifest after extirpation of a homoiogenous tumor does not 

 affect an autogenous tumor growing at the same time in the bearer of the 

 homoiogenous tumor, nor is the extirpation of an autogenous tumor followed 

 by immunity against inoculation with a homoiogenous tumor. 



The observation of the writer and of Jensen, that in animals in which 

 a first inoculation of a homoiogenous piece of tumor was not followed by 

 tumor formation, a second inoculation of a homoiogenous piece was also 

 unsuccessful, suggested to Jensen the idea that as a result of the first inocu- 

 lation immune bodies developed in the animal, which protected it against a 

 second inoculation, and that the phenomena apparently attributable to natural 

 immunity did in reality represent an acquired immunity. Subsequently, it was 

 observed however that under the conditions of Jensen's experiments immune 

 bodies cannot be demonstrated in the blood of the inoculated animal. Jensen's 

 work was the starting point for the investigations of Ehrlich and Bashford, 

 and their collaborators. Ehrlich and Apolant, extending to natural immunity 

 against transplanted tumors their conception of natural immunity against 

 microorganisms, assumed that specific X substances are needed to allow, in 

 a certain host, the growth of bacteria as well as of tumor cells. If there is 

 an insufficient amount of such an X substance present, a state of athrepsia 

 exists in the host as far as the microorganisms or cancer cells are concerned 

 and they are therefore prevented from growing in this host. Other investiga- 

 tors have attributed the natural immunity against transplanted tumors to the 

 action of lymphocytes, and this factor they held responsible also for the de- 

 velopment of an active acquired immunity against canter. Thus, in the case of 

 the Rous chicken sarcoma it was observed that in naturally immune fowl 

 lymphocytes collected around the tumor transplant ; it resembled in this respect 

 transplanted normal tissue, where likewise lymphocytes play a significant role. 



When it was found that it is possible in a certain percentage of animals, 

 which varies in different cases in accordance with the kind of tumor or host 

 used, to produce an active immunity through inoculation of normal tissues 

 or of certain kinds of tumor tissue, the view was expressed by Russell that 

 all natural immunity against tumor grafts is in reality a manifestation of 

 active immunity, due to the absorption of a certain amount of the inoculated 

 piece, which thus acts at the same time as an antigen. Whether an animal 

 proved to be naturally resistant (immune) or not depended therefore upon 

 its ability to develop an active immunity. This conclusion of Russell, which 

 represents an extension of Jensen's view, was very widely accepted and has 

 found expression even in recent literature. However, while active immunity 

 undoubtedly plays an important role in determining the fate of transplanted 

 tumors, this interpretation does not explain why certain individuals should 

 develop an active immunity, whereas others are not able to do so, and this is 

 the important point which needs to be elucidated. In the case of normal tissues 

 we have seen that such an interpretation would be inadequate. Here, the 

 primary relation between the organismal differentials of host and transplant 

 is the determining factor, and tumor tissue has retained in many essential 

 respects the characteristics of normal tissues, with the addition of certain 



