INDUCTION OF TOLERANCE BY PARABIOSIS 34I 



vicious circle may lead to the extermination of host lymphoid 

 tissue and replacement thereof by donor cells. By the above 

 process gradual " desensitization " of the host may take place with 

 the concurrent estabhshment of specific tolerance. 



However enticing the idea of complete replacement of the 

 immune system of the host had at first appeared, it soon became 

 evident that replacement in the anatomical sense of the word was 

 a goal very difficult to attain if attainable at all. That a tolerant 

 chimera may contain immunologically active cells of both host 

 and donor origin is revealed by alternating immune crises des- 

 cribed elsewhere (Nakic et ah, 1961) ; a tolerant parabiont may 

 recover from "parabiotic disease" with the graft soon showing 

 necrotic patches — a host-versus-graft reaction. If regeneration of 

 the graft takes place, the host may again show symptoms of 

 "parabiotic disease". Such alternating crises may frequently 

 recur. It would seem that tolerance is not a static phenomenon 

 but would be maintained by a very dynamic balance struck 

 between host and donor cells. A favourable donor/host cell ratio 

 might maintain tolerance indefinitely. A shift in balance towards 

 the host side would cause abolition of tolerance while a decrease 

 in the activity of host cells would result in fatal graft-versus- 

 host reaction. 



Summary 



Short-term parabiosis (5-6 days) between adult bilaterally 

 incompatible albino rats is followed by non-specific inhibition of 

 immune reaction which may gradually progress to specific 

 tolerance. 



In one strain combination, tolerance is associated with "para- 

 biotic disease", a fatal condition considered to be a graft-versus- 

 host reaction. In another strain combination, specific tolerance 

 can be induced without manifest signs of parabiotic disease unless 

 one of the partners is preimmunized. The first cross-graft may be 

 partially rejected but the second graft from the same donor one to 



TRANS. — 12 



