PROCESSES OF IMMUNITY 161 



transplants and in the preservation of the latter ; under these circumstances it 

 was very difficult to recognize the possible presence of slight effects of a first 

 on a second graft. However, as a rule, in accordance with expectations, the 

 reactions against the first and older graft were more severe, on the average, 

 than the reactions against the second graft. Furthermore, thyroid gland and 

 cartilage and fat tissue, which were the tissues transplanted in the majority of 

 cases, again behaved in a corresponding manner in the individual experiments. 

 It was likewise noticeable that the same host reacted with different degrees of 

 severity against homoiogenous transplants which had originated in different 

 donors, and different hosts seemed to differ in the severity of the reactions 

 against the same donor. There was in a number of instances, in the same host, 

 a correlation between the fate of the first transplant examined 33 days after 

 transplantation and of a second transplant examined after 12 days. When the 

 first transplant was relatively well preserved, or when the reaction against it 

 was severe, also when there was much lymphocytic infiltration in the first 

 transplant, similar conditions were found in the second transplant. This indi- 

 cates that the degree or reactivity of the host against homoiogenous differen- 

 tials was one of the factors that determined the results of the transplantations. 

 From these experiments we may therefore conclude that immune reactions in 

 all probability are not of paramount importance in determining the reactions 

 against homoiogenous transplants, but on the other hand, we cannot exclude 

 the possibility that slight immunizing effects may be exerted by a first on a 

 second homoiogenous transplant. 



The primary factors determining the fate of transplants are, then, the 

 differences in the individuality differentials of host and transplant, these 

 differentials being preformed and giving rise to relatively slow and mild, but, 

 in many instances, gradually accumulating primary reactions against the 

 transplants. The reactions of the host against the transplant set in as soon as 

 the latter has entered into organic connections with the host and as soon as the 

 individuality differentials have had a chance to diffuse from the transplant 

 into the host and here to set in motion a response on the part of the various 

 tissues. 



The investigations of M. S. Fleisher agree with our conclusions. He found 

 that immunization of an animal by homoiogenous tissues did not in any definite 

 way modify the course of the typical reaction against homoiogenous trans- 

 plants. Conditions were different in the case of heterogenous grafts ; here as a 

 result of immunization there was an increased accumulation of polymorpho- 

 nuclear leucocytes around the graft in the first few days following transplanta- 

 tion, as well as a delay in the ingrowth of fibroblasts into it, and furthermore, 

 a reduction in the slight growth processes which may take place in a heterog- 

 enous host; but these differences between immunized and normal hosts were 

 only transitory ; they soon subsided. These experiments are furthermore in 

 agreement with the view that heterogenous tissues are more efficient sources of 

 antigens than are homoiogenous ones, because the former are more strange 

 and are therefore more prone to initiate immune reactions. However, it seems 



