Immunity of Rats towards Jensens Rat Sarcoma. 29 



Further, as will be discussed later, acquired immunity appears to depend 

 on a process similar to that which occurs in disappearing tumours, but in 

 this case the control of growth begins at once, so that no proliferation of the 

 sarcoma cells occurs. 



After inoculation, the majority of rata subsequently become immune ; in 

 some *the immune condition appears soon after inoculation, in which case 

 the tumour resulting from the first inoculation only reaches a small size ; if 

 the onset of immunity be further delayed, the tumour may reach a large size 

 before regressing. 



The tumour may never regress, but only the rate of growth become less, 

 when the rat is found to be nearly always unsusceptible. 



It follows that the onset of unsusceptibility, and the power to inhibit the 

 growth of an established tumour, are closely associated. 



It has, however, been seen that an animal bearing a progressively growing 

 tumour, in which, therefore, no measurable inhibition of growth is occurring, 

 may be either susceptible or immune. This might be taken as evidence 

 that the two processes are not identical ; on the other hand, a force which is 

 sufficient to kill a few sarcoma cells struggling to establish themselves may 

 have no measurable effect upon a large, well-established tumour. An 

 alternative explanation has been given, on the view that a concomitant 

 immune condition is produced by the re-inoculation itself, which is sufficient 

 to prevent its taking, but insufficient to affect the progressive tumour. As 

 will be seen later, there is considerable evidence that the two conditions — 

 imsusceptibility and the power to inliibit growth of an established tumour — 

 are identical (see p. 30). 



Both of the conditions, susceptibility and immunity, are generalised ; no 

 matter where inoculation be made, the graft will take or fail to grow, as the 

 case may be. 



Immunity depends upon the ability of the animal to prevent the growth 

 of, and to destroy, sarcoma cells when introduced into its body. This 

 condition has been found to be associated with changes in the spleen, 

 in respect of the number of lymphocytes and plasma cells present ; never- 

 theless, it has not been possible to define exactly the relation between these 

 factors. There is, however, other evidence that the spleen plays some part, 

 viz. : (1) When mixtures of spleen and tumour are inoculated, the use of 

 immune spleen causes a greater delay in growth than in the case of normal 

 spleen. (2) If immune animals be splenectomised, the microscopic study of 

 grafts shows that, some growth of the sarcoma occurs. (3) If immune 

 animals be splenectomised, and at the same time a large dose of sarcoma be 

 given, in a few cases measurable tumours result. (4) If immune animals 



