408 GENERAL DISCUSSION 



adaptation), this could explain all the discrepancies which have 

 been observed. 



Medawar: Dr. Voisin, could you give the evidence that has led 

 you to believe that tolerance of cellular antigens is a totally differ- 

 ent phenomenon from tolerance of serum proteins, as in Richard 

 Smith's experiments, for example? I am not saying that they 

 aren't different, but I w^ould like to know your reasons for thinking 

 they are. 



Voisin: It seems to me that it would make things clearer if the 

 contrast was made, not between cellular antigens and serum 

 proteins, but between living cells and chemically defmed antigens. 

 It then becomes easier to visualize that tolerance of living cells is 

 probably a phenomenon different from and more complex than 

 tolerance of chemically defmed antigens. 



The main fact that led me to think that this must be different is 

 that when we use living cells they have to survive in order to in- 

 duce tolerance. Of course chemically defmed antigens have to 

 "survive", that is to stay, in the animal, in order to maintain 

 tolerance, but the cells injected in order to induce tolerance do 

 more than act as a permanent source of antigens ; most usually, 

 they are either immunologically competent cells or cells capable 

 of becoming immunologically competent (and I quite agree with 

 Dr. Nakic when, speaking of injection of embryonic cells, he 

 supposes that these cells will become immunologically competent 

 after a few days spent in the host). The cells usually react against 

 the host, and when the reaction takes the form of a homograft 

 rejection then it results in runting. Now, since we usually inject 

 immunologically competent cells, since these cells are homologous, 

 and able to react against the host, why do they not always react 

 against the host ? Why is it so often apparently harmless to the 

 host, if not because there is something protecting the host against 

 this immunological rejection reaction ? In the first days of life this 

 protective activity cannot come from a reaction of the host be- 

 cause this immature host is still unable to fight, so to speak, the 



