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injection course the animals were bled, and the serum rat-crythrocyte 

 agghitinin titres are shown in Table 1. Instead of becoming tolerant, the 

 mice developed high antibody titres. We also attempted to save irradiated 

 mice with embryonic tissues in the form of whole-embryo emulsion. About 

 20 per cent of the mice which were given 750 to 850 r followed with a 

 whole-embryo homograft, survived for at least 3 months. When these mice 

 were given a course of injections of foreign erythrocytes in an attempt to 

 make them tolerant, the majority developed antibody. This could mean 

 either that transfer to the adult environment favoured rapid maturation of 

 the embryonic cells, allowing these to make antibody; or else that the lethally 

 irradiated animals' own cells recovered rapidly enough to give an adequate 



Table 1. Antibody production in lethally irradiated mice 



immune response. Considering our previous results on the transfer of 

 normal neonatal spleen cells stimulated in vitro to X-irradiated adult re- 

 cipients, and the results of others, we consider the former hypothesis the 

 more likely. In any case, it is clear that tolerance to foreign erythrocytes 

 cannot be induced under these circumstances. 



This paper has covered a lot of ground, and has made brief reference to 

 several different experimental projects. The final interpretation of much of 

 the work awaits further experimentation. However, it is clear that far more 

 factors influence the production of antibodies than is generally realized. 

 The initiation of the process can only take place in adult animals, or in an 

 environment packed with adult cells. Conversely, immature cells can make 

 antibody when transferred to an adult environment. Antibody-producing 

 cells can be serially passaged several times and continue to function, but 

 there is no evidence that specific antibody-forming clone can be isolated in 

 this way. Therefore, there is still no experimental proof of Burnet's clonal 

 selection hypothesis, or of its modifications. However, a cellular immuno- 

 logical memory has been shown to exist, and numerous techniques have 

 been worked out for investigating the genetic implications of this pheno- 

 menon. The use of whole-body irradiation in depressing the immune 

 response has been a most valuable tool, and we feel that this branch of 

 radiation biology will be a productive one for many years to come. 



The authors wish to express their sincere thanks to Sir Alacfarlane Burnet for his 

 interest in this work and to the staff of the Peter MacCallum Cancer Clinic, especially 

 Miss Betty Rose, for performing the irradiations. 



242 



