THE UNIQUENESS OF THE INDIVIDUAL 



the transformation which leads to tolerance were closely akin 

 to the ''training'' of bacteria to metabolize unaccustomed food- 

 stuffs or to resist the action of inhibitory drugs. I think this 

 interpretation is plausible, because to secure a state of toler- 

 ance it is probably not sufficient merely to confront an embryo 

 with antigens; the antigens must persist, and continue their 

 educative action, well into the period in which, under normal 

 circumstances, the young animal would have become immuno- 

 logically mature. Fortunately, we need not commit ourselves 

 to any particular theory of the mechanism of tolerance before 

 examining its biological implications, which are various, 

 though I can consider only one or two. 



The phenomenon of tolerance requires one to think anew 

 about the nature of the relationship between a mammalian 

 mother and her unborn young. Except in highly inbred animals, 

 a foetus has a different genetic and therefore a different anti 

 genie constitution from its mother. It is therefore an antigenic- 

 ally foreign body, a kind of foreign graft. Why then does it not 

 immunize the mother, with consequences disastrous to itself.'^ 



Haemolytic disease of the newborn is evidence that this does 

 happen sometimes; that it happens very seldom is due to the 

 extraordinarily efficient insulation of the mother from her 

 unborn young. Under normal circumstances, no particles as 

 large as cells, and probably no molecules as large as nucleo- 

 proteins, could possibly cross the placental barrier from the 

 foetus into the mother*'s blood. This arrangement provides 

 against the danger that the foetus should immunize the mother, 

 but this is not the only immunological danger, and a one-way 

 control of traffic between foetus and mother is not enough. If 

 particles a-s large as cells could pass from the mother to the 

 foetus, then infective organisms or the antigens manufactured 

 by them could also do so, and although maternal antibodies 

 might keep infection in check, that would not prevent the 

 antigens of micro-organisms from damaging, perhaps irrepar- 



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