270 DISCUSSION 



runt survivors in the presence of hyperimmune anti-A serum is at best 

 80 to 90 per cent. Nevertheless, despite the lack of all-or-none results, 

 the evidence from cytotoxic isoantibody tests does not support the 

 assumption that a substantial number of donor lymphocytes are re- 

 phcating and leaving descendants. 



Simonsen: I too have seen Go wans' evidence and I have been very 

 much impressed by it. But I think we also should quote here the evi- 

 dence of A. J. S. Davies and S. M. A. Doak (i960. Nature [Loud), 187, 

 610) with the chromosome marker technique, which shows that 

 virtually none of the cells in mitosis, in the enlarged spleen, are of 

 donor origin. As far as the mouse goes, we would probably all agree 

 with you that the organ enlargement is mostly or wholly due to a host- 

 cell prohferation. What is really mysterious is not the origin, but the 

 role of the pyroninophihc cell, which is proliferating at a tremendous 

 rate in these organs, sometimes constituting more than 50 per cent of 

 the entire cell population. Some of them, as indicated by Gowans' 

 findings, must be of donor origin, butthe vast majority of the pyronino- 

 phihc cells, if we can accept Davies and Doak's results as being valid for 

 mice in general and not only for their own strains, must be of host 

 origin, and nobody knows what they are doing. 



