PROTECTION AGAINST RUNTING 321 



being elaboration of circulating antibodies against antigens closely 

 related to transplantation antigens (Gorer, 1957, 1958) (and 

 known as enhancing antigens). These antibodies are able, when 

 present at a certain level, to promote immunological facilitation 

 or enhancement. The fate of the homologous injected cells or the 

 homologous grafted tissues is likely to depend to some extent on 

 the predominance of one of the two reactions, rejection or 

 facilitation. But, although a predominant rejection reaction is a 

 defmitive phenomenon leading to the death of the graft, a 

 predominant facilitation reaction, helping the graft to survive, 

 is a precarious state that may be interrupted at any time. These 

 considerations may help in understanding the events which 

 follow an intravenous injection of immunologically competent 

 cells into a homologous newborn mouse (Voisin, 1962) : during 

 severals days the newborn is not immunologically competent; it 

 provides only a nutrient medium and an antigenic stimulus to the 

 injected cells. The immunological reaction of these cells is very 

 likely to follow the general scheme of a two-headed reaction: 

 rejection reaction and facilitation reaction; a competition can 

 take place between these two reactions for rapidity of appearance 

 and/or for intensity. If the rejection reaction of the injected cells 

 toward the newborn predominates over the facilitation reaction, 

 the animal will be runted and eventually die ; if the facilitation 

 reaction strongly predominates and/or precedes the rejection 

 reaction, the animal will be protected and will survive — and this 

 seems to be perfectly possible owing to the similarities between 

 embryonic and tumour tissue (Hamperl, 1956; Bernhard, 1961); 

 if the facilitation reaction predominates "weakly", the animal will 

 be partially or tardily runted. This interpretation is supported by 

 the reported fmdings concerning runting and protection against 

 it, particularly by the following facts: that runting, albeit being 

 due to the same basic phenomenon as graft rejection, has not the 

 same precise chronology, even using highly inbred strains of 

 mice: it is often delayed or incomplete or does not exist at all. 



