GENERAL DISCUSSION 4II 



cytes and skin grafts (Hasek, M. and Hort, J. [i960]. Nature {Lond.), 

 186, 985; Stark, O., Kren, V.and Frenzl,B. [1961]. Nature {Lond.), 

 190, 291) ; however, this might be due to the fact that these two 

 types of response are not induced by completely identical antigens. 



The other question is that it is extremely difficult to calculate an 

 equivalent dose of replicating antigen for inducing tolerance in 

 newborn and adult animals. The immaturity of the newborn, 

 non-reactive recipient provides an excellent proliferative advan- 

 tage to the inoculated living cells whose actual dose is therefore of 

 less significance here, whereas in an adult host an "equivalent" 

 inoculum (per unit body weight, for example) cannot multiply 

 (at such a rate at least) for it is reduced by immune elimination. 

 For example, such a quantity of fresh blood as i per cent of the 

 recipient's body weight can induce tolerance in newborn duck- 

 lings both to skin graft and erythrocytes whereas in adult ducks 10 

 per cent body weight of blood induces tolerance to erythrocytes 

 but is insufficient to do so with regard to skin (Hasek, M. and 

 Puza, A. [1962]. Folia hiol. {Prague), 8, 54). 



In other words, the economy with a given dose of reproducing 

 antigen is quite different in newborn and adult recipients. 



Medawar: When I opened this conference, I made some remarks 

 about terminology. One of the remarks I made was that it might 

 be desirable to use a non-committal term like "promotion" to 

 refer to prolongations of the life of homografts brought about by 

 immunologically specific but otherwise unknown means. Some- 

 one has since pointed out that the term "promotion" is pre- 

 empted for the behaviour of tumour cells, so I am afraid "pro- 

 motion" is ruled out. I was attracted by the term "facilitation", 

 but it is too late to persuade anybody to use that now. 



I don't want to review the transactions of the conference, but I 

 would like to say just a word about the study of antigens. In an 

 absolute sense nothing very spectacular has emerged so far from 



