THE UNIQUENESS OF THE INDIVIDUAL 



after birth — and has been extended to other mammals and 

 to birds. 



It follows, then, that the *"homograft reaction'' can be com- 

 pletly abrogated; the problem of making animals completely 

 tolerant of foreign tissue is not, as I had once feared, insoluble. 

 But it must be said at once that it is hardly feasible to apply 

 this technique to human beings — not so much because it would 

 require interference with an unborn baby, though that objec- 

 tion is grave enough, as because the state of tolerance is 

 absolutely specific. The white mice I referred to above, made 

 tolerant of CBA grafts by injecting them before birth with CBA 

 cells, invariably reject skin homografts from other, unrelated 

 donors; so likewise a human foetus injected with (for example) 

 blood cells from Mr Smith (il s''agit ici de pseudonymes) could 

 not be expected to accept homografts in later life from anyone 

 except Mr Smith himself. 



Among the multitude of experiments which my colleagues 

 and I have done in course of our analysis of the phenomenon of 

 tolerance, two only will be singled out for special mention. The 

 first is that the state of tolerance does not discriminate between 

 the different tissues of a single individual; an injection of blood 

 into the embryo will cause a tolerance of skin, an injection of 

 spleen cells will cause tolerance of a graft of the cortex of the 

 adrenal gland, and so on. If a tissue A, injected into an embryo, 

 is to cause tolerance of some other tissue B, grafted later in 

 life, then clearly B must contain no antigen that is not also 

 present in A. (If B had some antigen peculiar to itself, there 

 is no reason why it should not go into action and immunize 

 the host.) This condition appears to be fully satisfied when A 

 and B are any two different tissues from the same individual. 

 It follows that all the living tissues of a single individual must 

 have the same antigenic composition, w^hich is exactly what we 

 should expect if the antigens do indeed consist of chromosomal 

 matter. This is of great practical importance. In attempting 



178 



