GENERAL DISCUSSION 



Brent: I wonder if we could discuss the idea that in order for 

 tolerance to be operative, there must be some measure of graft- 

 versus-host reaction. It seems to me that there are a number of 

 instances in which it can be shown very clearly that it is possible to 

 have tolerance without any overt signs of a graft-against-host 

 reaction. One example is the injection of F^ hybrid cells into new- 

 born animals. Another example is the injection of embryonic 

 tissues into animals, or Dr. Hasek's parabiosis between chicken 

 embryos. In such instances it is impossible to invoke the hypo- 

 thesis which Dr. Nakic favours and on which I think his whole 

 interpretation of tolerance in adult parabionts depends. I think 

 we might well discuss this a little further. 



Nakic: I should like to make a distinction between overt signs 

 of graft-versus-host reaction (clinical manifestations of the disease) 

 and microscopic fmdings. We may have tolerant parabiotic 

 animals which show no clinical signs of graft-versus-host reaction 

 whatsoever, but if you analyse the spleens or the lymph nodes of 

 these animals you will always fmd "blotting-out" of the normal 

 structure. I would like to know whether anyone has ever seen a 

 tolerant animal where homologous immunologically competent 

 cells have been used to induce tolerance, in which the spleen and 

 lymph nodes do not show at least some derangement from the 

 normal. 



Medaxvar: I do not think that has really answered Dr. Brent's 

 question. You are asking if there is ever an absence of runt disease 

 symptoms when you inject immunologically competent cells. 

 Dr. Brent's question turns on the injection of cells which are not 

 competent. 



Billingham: Dr. Brent's argument is very compelling to me. 



404 



