PROTECTION AGAINST HUNTING 319 



C. How can passive facilitation counteract runting? 



This question may remain without an answer for some time. 

 There are only two things that we can say at present: first, that 

 one cannot see any reason to consider the operative mechanism 

 as different from the one at work in conventional immunological 

 enhancement (Kaliss, 19571?); ^i^<i> secondly, that the "afferent 

 hypothesis" (Billingham, Brent and Medawar, 1956) does not 

 seem able, alone, to take into account the fact that reinjections of 

 facilitating serum result in an additional protection of newborns — 

 a fact which is consistent with the observation that enhancing 

 sera can protect homologous transplanted tumours, even when 

 these sera are injected several days after transplantation (Kaliss, 

 1957^). After years of study, the question is still open to discussion 

 as to whether enhancing antibodies have a peripheral action, 

 coating the antigens and preventing them from reaching, un- 

 modified, the immunological centres and/ or protecting them 

 against an already established immunological reaction, or a 

 central inhibitory action on the immunological centres themselves : 

 it seems indeed possible that a specific antibody might be able to 

 prevent the immunological centres from responding to an 

 antigenic stimulus (toward the corresponding antigen), especially 

 in the direction of delayed hypersensitivity. In any case, a precise 

 knowledge of the mechanism of facilitation would be of great 

 theoretical and practical value. 



D. Is active facilitation the normal process leading to 



protection against runting? 



It was mentioned at the beginning of this report that one of the 

 most puzzling questions was: why do immunologically com- 

 petent homologous cells injected into newborn mice not always 

 react immunologically against the newborns or, if they do, why is 

 the reaction so often well tolerated by the host ? There are only 

 two theoretical possibihties : either the newborn animal reacts 



