IMMUNOLOGICAL COMPETENCE OF SMALL LYMPHOCYTES 259 



appeared and behaved quite like small lymphocytes incapable of 

 mitosis. None of the evidence obtained, ranging from dosage 

 experiments to tests for chimerism and attempted tritiated- 

 thymidine labelling, suggests that a significant number of cells 

 capable of multiphcation was present among the injected cells. 

 A recent investigation by Cole and Carver (1961) demonstrates 

 that lymphocytes, but not granulocytes, are capable of procuring 

 transplantation disease in mice. However, they attribute the 

 immunological reactivity to prohferating large and medium 

 lymphocytes, and not to small lymphocytes. Although our data 

 clearly indicate that small lymphocytes are immunologically 

 competent cells, it remains to be determined whether these are the 

 only mischief-making cells when mixed populations of lymphoid 

 cells are injected. In apparent conflict with the results of Siskind, 

 Leonard and Thomas (i960), both Simonsen (i960) and Dineen 

 (1961) have found that chimeric spleen cells from runted mice 

 usually fail to produce further runt disease on passage to new 

 hosts. Although the latter workers have interpreted their 

 fmdings in terms of acquisition of tolerance by adult graft cells, 

 these divergent results might all be attributable to gradual death 

 of the original donor lymphocytes present and/ or their failure to 

 multiply. 



The fmding that small lymphocytes are immunologically 

 reactive, but apparently produce no descendants, has an obvious 

 bearing on clonal selection theories of immunity. Recent work 

 essentially proves the developmental sequence of lymphocytes 

 to be large->medium-> small (Everett et ah, i960). Most investi- 

 gators now agree that large and medium lymphocytes are 

 actively mitotic, whereas small lymphocytes (except thymocytes) 

 are end cells that rarely or never divide (see Ciha Found. Symp. 

 Haemopoiesis^ i960). The notion that small lymphocytes are 

 effete is behed by their content of highly active RNA as well as 

 by their long hfespan. Although there is evidence that some 

 lymphocytes can, under certain conditions, turn into monocytes 



