EXPERIMENTAL LEUKAEMIA 



of lymphocytes and some features of the anamnestic responses 

 in immunology. Experimental study of the possibility of 

 inducing leukaemia or neoplastic change in other cells by 

 injection of nucleic acids is actively in progress in several 

 laboratories, and clarification of the situation can be expected 

 in the near future. 



4. Human leukaemia 



Returning to the topic of leukaemia in man, and concen- 

 trating on clonal aspects, we will be specially concerned to 

 look for evidence of step-like changes and of the increased 

 frequency of a further mutation on top of one that has already 

 induced proliferative change of a cell type. 



[a) Chronic myeloid leukaemia 



The best example is chronic myeloid leukaemia. It is not 

 very infrequent for this to be recognized by the accident of 

 a blood smear being examined routinely, before the patient 

 shows any definitive symptoms. In such instances the myelo- 

 cytic series is clearly proliferating abnormally — this is evident 

 both from the peripheral blood and from bone marrow 

 examination. It is never possible to be certain how a muta- 

 tional change is initiated, but it is particularly in regard to 

 myeloid leukaemia that the evidence incriminating ionizing 

 radiation is strongest. I find it impossible to conceive any 

 reasonably direct effect of small doses of radiation that would 

 not be due to a mutation impressed in the first instance on 

 an isolated cell. One could believe that even if appro- 

 priate damage were inflicted on a ' blast' cell, it might remain 

 inert until general or local conditions allowed it to initiate 

 a proliferative clone. It seems quite possible that this may 

 allow a multiclonal as well as a uniclonal origin for some 

 leukaemias. The relevant point, however, is simply that a 

 proliferative process of mutational origin has grossly altered 

 the number and distribution of certain clones of cells, but has 

 left them with many normal or near-normal characteristics 



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