DISCUSSION 41 



female is a valid criterion of presence of antigen ? If you take a com- 

 bination where you know there is an antigenic difference, do you then 

 get accelerated clearance of the foreign red cells ? 



Hassek: We have not tested this question systematically. 



Amos: Sachs and Heller published some negative results in the mouse 

 of the Y antigen on the red cells ; the antigen was present in spleen and 

 liver. 



To go back again to the question of enzyme inactivation : I wonder 

 whether the efficiency of the irradiated cells that Revesz used was partly 

 due to loss of enzymic activity in the cells. We have compared the 

 efficiency of 20 million X-irradiated versus smaller doses of living, and 

 also a formol-acetone extract and a citric acid extract preparation of 

 tumour cells to immunize against a challenge. We found that while 

 the X-irradiated were more efficient than the live cells, both the formol 

 extract and the citric acid extract, and especially the citric acid, gave us a 

 very great degree of promotion. 



Russell: We have tried to induce tolerance with /// vitro irradiated 

 splenic cells in the hope that we could produce tolerance without runt 

 disease (this would be the sort of thing that you were talking about). So 

 far we haven't been able to fmd any dosage which will eliminate the 

 production of runt disease without also ehminating antigenic activity. 



Hildemann : I wonder if Dr. Mitcliison's success in producing tolerance 

 in chickens with red cells might not be attributable to the fact that these 

 are nucleated red cells. In our own experience testing the antigenic 

 potency of the Syrian hamster red cells versus buffy-coat cells, the 

 degree of skin homograft immunity induced was proportional to the 

 number of blood leucocytes present in the inoculum. As Billingham 

 has pointed out, many of us have observed that a very small dose of 

 leucocytes is antigenically effective. I know of no method for obtaining 

 absolutely pure preparations of red cells to make an unequivocal 

 test. 



Davies: One can make a good separation of red cells and white cells 

 by centrifuging through a sucrose gradient. 



Brent: I think there may possibly be a way round the difficulty in this 

 running battle of Barrett versus the rest. In your experiments, Dr. 

 Barrett, you are using two strains in which there is some measure of 

 incompatibility — if I remember rightly, about 10 per cent of the animals 



