GENERAL DISCUSSION 4O9 



injected cells. This protective activity must then come from the 

 injected cells themselves. The only mechanism I can think of for 

 this protection is that the injected cells themselves react by pro- 

 ducing antibodies against the newborn, and that, at a certain level, 

 this antibody production will act as an enhancing factor and 

 prevent the rejection reaction, or protect the newborn against the 

 consequences of this rejection reaction. This becomes more and 

 more a real possibihty in view of the recent knowledge on 

 competition between serum antibodies and delayed hyper- 

 sensitivity. 



Brent: I would like to ask our expert on delayed hypersensitivity, 

 Dr. Lawrence, a question. Is a cell capable of giving the '* cellular " 

 response at the same time as it gives the serum response ? In other 

 words, can these two responses be ehcited at one and the same 

 time, or must one succeed the other ? 



Lawrence: The difficulty in giving a clear answer to the question 

 stems from the lack of an in vitro test for delayed hypersensitivity 

 and, so far, only an in vivo skin test to reveal its presence. Where 

 it has been looked for in relation to serum antibody formation, 

 delayed allergy may precede the appearance of detectable serum 

 antibody. Our complete ignorance of the cell or cells involved in 

 the induction, manifestations and transfer of delayed allergy 

 would also engender caution. The lymphocyte has assumed a 

 position of prominence in each phase of delayed allergy, but it may 

 be more reahstic to leave the matter open at this time. The answer 

 is that we do not know. 



Brent: Prof. Medawar and I have done an experiment in which 

 we injected allogeneic F^ lymphoid cells into adult mice. These 

 normally have a strongly sensitizing effect, but when we injected 

 them repeatedly we gradually produced a condition which very 

 much resembled that of desensitization — reduced responsiveness 

 to skin grafts in the presence of circulating serum antibodies. 

 This suggested that the response of the animals, which had initially 

 been that of a delayed sensitivity, had gradually changed over to 

 14* 



