PROTECTION AGAINST HUNTING 287 



cells and the absence of induction of tolerance. The second way, 

 which does not present this inconvenience, is to inject the newborn 

 with embryonic, non-immunologically competent homologous 

 cells which will eventually become tolerant to the host (Simonsen, 

 1957) or to inject a newborn mouse with hybrid parental spleen 

 cells, genetically unable to react against the newborn host. But 

 this is not always easy to do and it cannot be applied to non- 

 inbred animals. As a consequence, the usual way of inducing 

 experimental specific acquired tolerance is still to inject homo- 

 logous adult spleen cells, knowing that one has to pay tribute to 

 runting disease. 



From a theoretical point of view, today the problem is not so 

 much to explain why and how the homologous disease sets in, 

 but rather the reverse problem: since the injected cells are known 

 to persist indefmitely in the homologous tolerant host (Billingham 

 and Brent, 1959) ; since many of these cells are immunologically 

 competent; and since it is known that they are able to react 

 immunologically against the host, why do they not always do this ? 

 Or, if they do, why is the reaction so often well tolerated by the 

 host (Voisin, 1962) ? 



The present investigation was undertaken in an effort to answer 

 these questions. It is clear, indeed, that classical concepts of homo- 

 graft rejection reaction and of acquired immunological tolerance 

 do not provide a satisfying answer, unless they are modified in 

 some way. One of the required modifications would be the 

 possibility for some of the injected cells from an adult homo- 

 logous donor (presumably the "stem cells" which have embry- 

 onic potentialities) to become tolerant to the young recipient. 

 A second required modification would be that the injected 

 immunologically competent cells which have already started a 

 homotransplantation reaction against the young host be eliminated 

 in one way or another, the more interesting theoretical suggestion 

 in this respect being the concept of "exhaustive sensitization" 

 (Simonsen, i960). However, the very existence of the runting 



