284 DISCUSSION 



guinea pigs, using the tuberculin system in place of the homograft 

 system. Is this correct ? 



Lawrence: The transfer with leucocyte extract in animals was one of 

 the reasons for raising the point about sensitization and desensitization 

 occurring in parallel. In the experiments we did in inbred strains of 

 guinea pig, with you and Prof. Medawar, in attempting to hyper- 

 sensitize the animal with lymph node cells as homograft antigen, flare- 

 up of all the previous sites of injection of antigen occurred with sub- 

 sequent desensitization. Our approach in the guinea pig using the 

 tuberculin system viewed the failure to transfer with leucocyte extracts 

 in animals as not necessarily a result of species difference but rather 

 some quantitative aspectof the system which we don't fully understand 

 as yet. In most reported attempts at transfer with extracts the donor 

 animals have received one course of sensitization. Our notion involved 

 the possibility that repeated hypersensitization with tubercle baciUi 

 might allow the transfer of sensitivity with extracts. When we tried 

 this approach we found the apparently positive initial results few and 

 not consistently reproducible. We also have serious reservations about 

 the meaning of the scattered positive reactions occasionally observed in 

 our guinea pigs, in view of Chase's finding that leucocyte extracts alone 

 can enhance the reactivity of guinea pig skin to unrelated delayed 

 allergens. We do not beheve the use of extracts is an impossibiHty in 

 the guinea pig, but rather no one has yet hit upon the appropriate means 

 of accompHshing it. 



Medawar: We often speak rather ghbly about "hyperimmunizing" 

 animals against isoantigens but I don't know of any circumstances under 

 which one can hypcrsensitize them: for example, the repeated injection 

 of animals with foreign lymphoid cells doesn't hypersensitize them, it 

 hyperimmunizes them: they become, during the course of this process, 

 less sensitive. And I don't really know of a method of making animals 

 hypersensitive to transplantation antigens. If we could do that, I should 

 imagine it might be possible to reproduce your phenomenon in a 

 guinea pig. 



Lawrence: In respect of the phenomenon I would say that the capacity 

 to transfer tuberculin-type sensitivity with leucocyte extracts in the 

 guinea pig is still an unsettled question despite much negative evidence 

 having been secured. I would, however, stress that we surely do not 



