348 



DISCUSSION 



homologous foetus into a foetus or a newborn animal, phase I and 

 phase II will start at the same time (sometime around the second week 

 in mice) ; in that situation, the host will be in a better position and a 

 reciprocal enhancement would be the most likely consequence. It must 

 also be understood that this concept does not automatically preclude 

 other immunological phenomena from taking place. 



^^ ^^genic Stirr^^ j^ 



Injected cells, 

 tolerated 



(Facilitation predominates \ 

 over Rejection f- 



^ Mouse 

 .tolerant ) 



Fig. 5 (Voisin). Phase II (host versus cells). Predominance of the facilitation 

 reaction over the rejection reaction: cells persist; the mouse is tolerant. 



Finally, the main feature of the proposed hypothesis resides in that, 

 instead of considering immunological tolerance to Hving cells as a 

 passive state of immunological non-reactivity induced in immature 

 animals, it is considered as an active state of a two-way immunological 

 reactivity resulting in a delicate balance. 



Woodruff: We (Michie, D., Woodruff, M. F. A. and Zeiss, I. [1961]. 

 Immunology, 4, 413) have some experiments which point to conclusions 

 very different from those of Nakic about this takeover business. In 

 adult A mice which were made tolerant of CBA by an injection of 

 spleen cells at birth, we fmd that most of the immunologically com- 



