INDUCTION OF TOLERANCE BY PARABIOSIS 333 



where the number of tolerant animals and degree of tolerance 

 were about even in the two strains. We believe that this difference 

 is due to the greater genetic homogeneity of the strains used in 

 the present series. 



No clinical symptoms of "parabiotic disease" have been ob- 

 served in any of the tolerant Y59 parabionts. This is an interesting 

 finding in view of the high susceptibility of Y59 animals to 

 "parabiotic disease" in combination with W partners. However, 

 all the tolerant animals displayed at least some degree of lymphoid 

 atrophy as found by histological analysis of their spleens and 

 lymph nodes. 



Although the immune reaction in most grafts carried by tolerant 

 animals was just beginning at the time when grafts in control 

 recipients would have already shown an advanced degree of 

 epidermal breakdown, superficial scaling or multiple small 

 necrotic areas were observed in all recipients by the end of the 

 second week. These necroses either spread to involve the entire 

 graft or the process of destruction became arrested for a while 

 only, to be resumed after a few days. Such recurrent attacks 

 continued until complete destruction of most grafts took place. 

 In hosts displaying long-term tolerance, the immune reaction 

 slackened considerably during the fourth postoperative week. 

 As a result of this, the condition of the graft became rather stable, 

 but by that time most grafts were considerably reduced in size 

 and of some only a vestige remained. It was this apparent cessa- 

 tion of the immune reaction that prompted us to test both the 

 degree and the specificity of tolerance. 



The specificity and degree of tolerance induced in Y59 



parabionts 



Table II shows results of the experiment in which seven 

 tolerant parabionts were challenged with the second cross-graft 

 and a graft from a foreign strain unrelated to either parabiont. 



