288 GUY A. VOISIN AND RADSLAV KINSKY 



syndrome does not much favour a mechanism which would auto- 

 matically prevent that syndrome because of the overwhelming 

 quantity of homologous antigens encountered by the injected 

 cells. For this reason and on the basis of theoretical considerations, 

 it was thought of interest to see whether or not another pheno- 

 menon known in the field of transplantation would give a 

 clue to the questions under investigation ; namely the enhancement 

 phenomenon or phenomenon of immunological facilitation* 

 of the graft by the immunologically competent cells (usually of 

 the host) themselves. This phenomenon is well known (Casey, 

 1941 ; Snell, 1954; Kaliss, 1957^) : it allows tumours indigenous to 

 one strain of mice to grow successfully when grafted in an adult 

 homologous H-2 histo-incompatible mouse which was previously 

 actively or passively treated for enhancement. Active treatment 

 involves immunization with tissues (usually lyophihzed) from the 

 future donor's strain; passive treatment is reahzed by an injection 

 of serum from an actively pretreated animal. This phenomenon 

 of immunological facilitation seems to be a general phenomenon 

 since it seems to exist also in rats and rabbits. It applies to many 

 tumours, but not to all (Snell, 1957^). It seems to apply also to 

 normal tissues (Snell, 1957/j) in spite of two facts: first, that 

 passive transfer of immunological facihtation has not been 

 firmly demonstrated in cases of normal tissue grafts; and, second, 

 that immunological enhancement results only in some degree of 

 prolongation in the survival time of a normal tissue homograft. 

 This was to be expected because of the much weaker growth 

 potential of normal tissues as compared to tumour tissues. But this 

 concerns normal adult tissues ; one may wonder whether embry- 

 onic tissues might not react more like tumour tissues do. Actually 

 there are striking analogies between embryonic tissues and tumour 

 tissues (Hamperl, 1956; Bemhard, 1961). These analogies are of 



* "Immunological enhancement" and "immunological facilitation" will be 

 used indifferently with the same meaning. "Facihtation" seems a better term 

 because it can be apphed to normal tissues as well as to tumour tissues. 



