HOMOGRAFT SENSITIVITY IN HUMAN BEINGS 279 



Conclusions 



The transfer of homograft sensitivity in human beings confirms 

 the earher findings of Mitchison (1954) on the transfer of tumour 

 homograft sensitivity in mice; of BilHngham, Brent and Medawar 

 (1956) on the transfer of skin homograft sensitivity in mice; and 

 of Brent, Brown and Medawar (1958) on the "transfer reaction" 

 in guinea pigs. The abihty of DNAse-treated leucocyte extracts 

 to transfer homograft sensitivity in man is a striking departure 

 from the requirement in animal species for intact, viable cells to 

 achieve this effect. This discrepancy is believed to reflect more the 

 relative ease with which man may acquire and express sensitivity 

 to a variety of foreign materials, rather than any real species 

 difference. It would appear that the difference in the behaviour 

 of the cellular transfer system in homograft reactions arises from 

 the degree of sensitivity possible for various species (mouse, 

 guinea pig, man) to achieve, and the means of immunological 

 expression at the disposal of each (Lawrence, 1960^). 



The fact that transfer of homograft sensitivity in humans must 

 be done in genetically diverse outbred populations and the fact 

 that the individual specificity of homograft reactions is just com- 

 ing under scrutiny (Rapaport et al., 1960^) introduce inescapable 

 variables in evaluating the results achieved. Nevertheless, since 

 the effect of transfer on test grafts has been judged significant only 

 when compared to the behaviour of control grafts on the same re- 

 cipient, the role of such variables, although not fully eliminated, 

 has been greatly diminished. 



The unique attributes of the transfer of homograft sensitivity 

 in man which may prove useful in formulating concepts of at 

 least one mechanism of orthotopic homograft rejection are: 

 (i) The demonstrated effectiveness of leucocyte extracts and 

 various cell-free preparations in the transfer of delayed sensitivity, 

 a reaUty that affords an opportunity for identification and 

 characterization of transfer factor mediating these effects. (2) The 



