IMMUNITY IN TUMOR TRANSPLANTATION 417 



although it was conceded by some authors that also immune reactions, which 

 were analogous to those developing against embryonal tissues, may participate 

 in this process. Other investigators believed that the acquired resistance or 

 immunity against tumor transplants led to a deficiency in the ingrowth of 

 stroma from the host into the tumor. Under normal conditions the surrounding 

 host tissue supplies the tumor with blood vessels and a connective tissue 

 stroma ; but it was assumed that if the host has been made resistant or immune 

 against the transplant, it fails to provide this stroma. 



As to the lymphocytes, in the case of normal tissue transplants we found a 

 double significance of these cells: (1) Under certain conditions the strength 

 of the lymphocytic accumulation could be used as a quantitative measure of 

 the intensity of the reaction of the host against a strange individuality differ- 

 ential ; it served therefore as a standard with which to measure the difference 

 between the individuality differentials of host and donor, and, accordingly, 

 also of their degree of relationship or strangeness. (2) The lymphocytes, in 

 collecting around the transplant and invading it, were able to injure it if they 

 penetrated into it in dense masses. On the other hand, we did not find any 

 evidence for the further conclusion that the lymphocytes give off substances 

 which diffuse into the transplant and thereby damage it, an assumption that 

 was made by some investigators in the' case of tumor transplants. 



The role which lymphocytes play in the growth and retrogression of trans- 

 planted tumors seems to be similar to that seen in the case of transplanted 

 normal tissues. There is, however, one significant difference. While the marked 

 accumulation of lymphocytes around normal tissues and their invasion of these 

 tissues may lead to the injury and destruction of a considerable part of the 

 graft, in the case of a growing tumor the multiplication and expansive growth 

 of the tumor cells may be so active that the lymphocytes cannot overcome the 

 graft. Also, around retrogressing tumors the local accumulation of lympho- 

 cytes does not need to be very conspicuous. This condition accounts perhaps 

 for the fact that in the transplantation of tumors several investigators did not 

 attribute to the lymphocytes the role which we did in the grafting of normal 

 tissues, but they considered them, rather, as important agents in the production 

 of the general immunity which develops under various circumstances against 

 tumor transplants. This latter interpretation seemed also to be supported by 

 the observation that while an accumulation of lymphocytes may become 

 noticeable already after a first inoculation of a piece of tumor, it is more 

 accentuated and it appears more rapidly after a second inoculation, because 

 here the inoculation takes place in an animal in which immunization processes 

 have set in already as the result of the first inoculation. 



As stated, the activity of lymphocytes around a piece of tumor does not 

 need to be pronounced; this is true especially if the first transplanted piece 

 begins to grow actively soon after transplantation. An accumulation of lym- 

 phocytes was more marked in the early experiments of Burgess and Tyzzer; 

 however, these investigators did not study the local reaction around homoiog- 

 enous tumors, but around pieces of tumor which approached a heterogenous 

 character; and here polymorphonuclear leucocytes were as prominent as 



