PROTECTION AGAINST HUNTING 323 



logically non-competent partner (the homografted tumour or the 

 injected newborn). By doing so, one realizes at once that a new- 

 born recipient (immunologically non-competent partner) can 

 be passively "enhanced" or "facihtated" in such a way as to be 

 protected against an otherwise lethal rejection reaction on the part 

 of the injected cells (immunologically competent partner). This 

 passive enhancement of non-tumour tissues has been shown to be 

 unequivocal and sometimes dramatic, as it is with tumour tissues. 



B. Reasons making this phenomenon possible 



At this point, it is only possible to speculate upon the reason 

 why embryonic or neonatal tissues react to immunological faciH- 

 tation in a way closer to the reaction of tumour tissues than to 

 that of normal adult tissues. In other words, why do neonatal 

 tissues benefit from enhancement much more than normal adult 

 tissues do? We are tempted to think that surface phenomena 

 (including a high pinocytotic activity) common to tumour tissues, 

 embryonic tissues (and also stem cells) might be the basis for a 

 tentative explanation. If this were so one might predict that 

 stem cells should also be susceptible to enhancement, a fact which 

 would be very useful in the understanding of several phenomena 

 related to transplantation immunity and acquired tolerance to 

 living cells. 



III. Acquired tolerance and its enhancement 



The objective fact to be discussed is the significant increase in 

 the proportion of tolerant animals and, above all, in the quality 

 of tolerance, in the animals protected by facilitating serum as 

 compared to non-protected animals. To try to explain this result 

 would again require some knowledge of the nature and mechan- 

 ism of immunological facilitation-enhancement. Unfortunately 

 the operative mechanism is still poorly understood (probably as 

 poorly understood as the action of Bogomoletz serum is !) and a 



