DISCUSSION 



Miller:]. L. Go wans, B. M. Gesner and D. D. McGregor (196 1. Ciba 

 Found. Study Gp lO, Biological Activity of the Leucocyte, p. 32. London: 

 Churchill) have evidence that suggests that the small lymphocyte can 

 initiate a graft-versus-host reaction. It is true that small lymphocytes 

 cannot be labelled by a single injection of tritiated thymidine, but if 

 you give repeated injections or a continuous infusion of the radioactive 

 label to a rat, a larger proportion of small lymphocytes become 

 labelled. If such labelled cells are injected into an F^ hybrid, then 

 24 hours after injection, labelled small lymphocytes appear in the 

 splenic white pulp. But in addition there are other types of cells which 

 show labelling in their nuclei: some of these are in mitosis, others have 

 a thin rim of pyroninophilic cytoplasm, a pale nucleus and a single 

 nucleolus, and others are larger, with more abundant, densely pyronin- 

 ophilic cytoplasm and one or two prominent nucleoli. Is it possible 

 that the small lymphocyte, in order to initiate a graft-versus-host re- 

 action, has to transform into another type of cell such as the pyronino- 

 phihc cells described by Gowans and his colleagues? Have you any 

 evidence of such a transformation in your work ? 



Hildemami: The most recent evidence of which I was aware is con- 

 tained in the Ciba Foundation volume on Haemopoiesis of last year. 

 Everett et al. (cited in my paper) have demonstrated that the develop- 

 ment sequence of lymphocytes is large to medium to small, and that 

 the labelling that one may detect in small lymphocytes is attributable 

 to label that is carried over in the process of division, in going from 

 medium to small. Many investigators have of course argued that small 

 lymphocytes are capable of transforming into a variety of cells. My 

 understanding of the current consensus is that small lymphocytes may 

 under certain circumstances transform into macrophages or monocytes ; 

 evidence that they transform into any other type of cell is controversial. 



Medawar: I think everyone would be convinced by Gowans' latest 

 evidence, just cited by Dr. Miller, on the transformation of thoracic 

 duct lymphocytes when they are in the course of mounting a graft- 

 against-host reaction. A clear picture is building up of the lymphocyte 

 as a cell which does not proliferate as such because it is really the larval 



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