THE UNIQUENESS OF THE INDIVIDUAL 



to go on was Owen's discovery (p. 150) of red-cell chimerism in 

 twin cattle — the state of affairs in which cattle twins are born 

 with and long retain a mixture of each other''s red blood 

 corpuscles, presumably because they exchange blood-forming 

 cells in embryonic life. If the exchange had not occurred before 

 birth, but had been carried out artificiallv afterwards, then 

 the foreign blood-forming cells would quite certainly have been 

 recognized as such and destroyed by an immunological reaction. 

 The exchange of the cells before birth must somehow have pre- 

 vented the development of that faculty which would have em- 

 powered the twins to recognize each other*'s cells as not their own. 

 My colleagues and I have shown that Burnet and Fenner''s 

 prediction is true, without qualification, of the antigens which 

 are responsible for transplantation immunity. We too began 

 our work on cattle. In 1948, while attending an International 

 Congress of Genetics at Stockholm, I was invited by Dr H. P. 

 Donald to help to solve the important problem of distinguishing 

 with complete certainty between identical and non-identical 

 twins in cattle. In principle, nothing could be easier. Skin 

 grafts were to be exchanged between the twins a few weeks 

 after their birth. If the homografts survived, the twins could 

 be classified as identical; if not, as non-identical, i.e. dizygotic. 

 My colleague Dr Billingham and I, helped by two young officers 

 of the Agricultural Research Council, began what was to be a 

 few months'* work later on that year; as it turned out, the work 

 took three years to finish. We satisfied ourselves that the 

 reaction against skin homografts was no less vigorous in cattle 

 than in man or in laboratory animals, and that skin grafts 

 exchanged between cattle of the same or of different breeds, 

 or between dam and calf or vice versa, or between ordinary 

 siblings (brothers or sisters but not twins) were all destroyed 

 within a fortnight of their transplantation. But homografts 

 between almost all the twins survived, irrespective of whether 

 the twins were identical or dizygotic. There could be no mistake 



176 



