DISCUSSION 



345 



During the first phase the newborn is not immunologically compe- 

 tent; it provides only a nutrient medium and an antigenic stimulus to 

 the immunologically competent injected cells. In turn, these cells react 

 against the host and they can do so in two ways, leading respectively to 

 a rejection reaction and a facilitation reaction. A competition takes 

 place between these two reactions for rapidity of appearance and/or 

 intensity. 



Nnl\ge^">c Stimufus 



Rejection predominates over Facilitation 



Fig. 2 (Voisin). Phase I (cells versus host). Predominance of the 

 rejection reaction over the facihtation reaction, leading to runting. 



During the second phase, that competition can continue, but now 

 the young animal becomes immunologically competent and starts to 

 respond to the antigenic stimulus of the injected homologous cells; 

 this immunological response can also be exerted in two ways — rejection 

 and facilitation. Here again a competition takes place. 



The direct logical consequences of the hypothesis are rather obvious. 

 During phase I, if the rejection reaction of the injected cells towards the 

 homologous newborn predominates over the facilitation reaction, the 

 animal will be runted and eventually die (Fig. 2) ; if the facilitation 



