416 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



function of transplanted tumor cells and normal cells and of injected eryth- 

 rocytes, which latter elicit the formation of hemagglutinins and hemolysins. 

 The differences which do exist seem to be mainly of a quantitative nature, the 

 immune substances, which are formed after transplantation of living normal 

 tissues, being weaker. There is, perhaps, the additional difference that the 

 growing tumor seems to be able to absorb and to neutralize very effectively 

 the immune substances circulating in the host, while normal tissues and eryth- 

 rocytes do not do so. The proportion of neutralized and non-neutralized 

 immune substances and the power of resistance of the tumor tissue to the ab- 

 sorbed immune substances seem to vary in the case of different tumors, and it 

 is largely these variations which are probably responsible for the differences 

 in the types of immunity which develop after transplantation of tumors into 

 hosts whose organismal differentials differ from those of the tumor. However, 

 as we have stated already, there are indications that not only the organismal 

 differentials, but also other constituents of the tumor cells, may give rise to 

 states of immunity and to the production of immune substances ; some of these 

 data we shall discuss in the last part of this chapter. 



The presence of immune substances in the circulating bodyfluids of an ani- 

 mal actively immunized against a transplanted tumor is perhaps suggested also 

 by the demonstration of the presence of substances in the circulating body- 

 fluids which enable a tumor graft to grow in an animal belonging to a strain 

 unfavorable to the growth of a particular type of tumor. This has been dem- 

 onstrated by means of parabiosis, if an individual belonging to a strain favor- 

 able to the growth of the transplanted tumor was joined to an individual 

 normally resistant to the growth of the inoculated tumor. Substances supplied 

 by the first partner enabled the tumor to grow in the second otherwise unsuit- 

 able partner. We have referred already to experiments of this kind by 

 Zakrzewski and of Cloudman in a preceding chapter, when we discussed 

 factors in the growth of transplanted tumors. However in this case we have to 

 deal with substances favoring tumor growth while here we are concerned with 

 substances inhibiting tumor growth. 



(6) Cellular reactions of the host against the tumor transplants. We have 

 discussed evidence which tends to prove that under various conditions immune 

 substances directed against the individuality or species differential of the 

 transplanted tumor may develop in the host. But, in addition, certain types of 

 host cells react against the transplant, and these reactions manifest themselves 

 locally around the graft as well as in the circulation of the host. The local 

 reactions consist, above all, in the accumulation of lymphocytes, but also 

 polymorphonuclear leucocytes, connective tissue and blood vessels may take 

 part in these processes. 



Simultaneously with the early studies of the role of lymphocytes in trans- 

 plantation of normal tissues there began the study of the role of various types 

 of leucocytes, including lymphocytes, in the reactions of the host against a 

 tumor. But the cellular changes against transplanted tumors were interpreted 

 as the local manifestations of a general immunity against the tumor growth. 

 This immunity was considered as distinct from other types of immunity, 



