THE UNIQUENESS OF THE INDIVIDUAL 



appear, and so pay back the still outstanding balance of his 

 mortality. 



All known chimeras, then, are twins, and all such twins have 

 been found to accept skin homografts from each other for as 

 long as the state of chimerism endures. So when I say that 

 embryos will accept homografts because they are not yet old 

 enough to have acquired an immunological conscience, to have 

 learnt the difference between what is native to them and what 

 is foreign, that is only half the story. The other half is that 

 foreign cells introduced into an embryo affect it in such a way 

 that it may never acquire the power to recognize the cells as 

 foreign, and may accept them as its own. It will accept, more- 

 over, not merely the cells which gained access to it as an 

 embryo, but any cell of the same genetic composition that may 

 be transplanted to it in later life. This is the origin of the 

 concept of acquired immunological tolerance, of which I shall 

 say more in Section 5 below. 



3. MECHANISMS 



Skin homografts are destroyed by an immunological re- 

 action, that is by a process fundamentally akin to that which is 

 provoked by bacterial, viral or cellular infections, or by the 

 injection of foreign proteins or polysaccharides. For all the 

 clinical good-will and perhaps even mortal urgency that accom- 

 panies their transplantation, skin homografts are treated as if 

 they were a disease of which their destruction is the cure. 

 (This outdoes Erewhon: the disease is beneficial, the cure does 

 harm.) Transplantation immunity therefore resembles the 

 allergies and hypersensitivities and serum sickness, and trans- 

 fusion accidents and haemolytic disease of the newborn, in 

 being an immunological reaction-gone-wrong; and so far have 

 we travelled from the days when all immunological reactions 

 were supposed to be necessarily beneficial (however mysterious 



152 



