MESENCHYMAL CELLS 



Medawar (1957) has recently suggested that the essential 

 function of the lymphocyte is as a carrier of nucleic acids. 

 He bases this largely on the evidence of Otteson (1954) and 

 Hamilton (1956) that lymphocytes labelled with ^zp give 

 indications that some of the cells (or their nucleic acids) have 

 lives of 100 to 300 days. There is no doubt that a circulation 

 of lymphocytes exists but there is known to be a constant 

 wastage and it is hard to accept so long a life for the intact 

 cell. If in lymph nodes or elsewhere the nucleic acids of 

 effete cells were used as the source of nucleic acids for the 

 new cells of the germinal centres with mobilization of their 

 phosphate, a more satisfactory interpretation would be avail- 

 able. Medawar considers that there is no evidence that 

 functionally genetic nucleic acid is taken into the new cells, 

 though Hamilton toys with this possibility. 



From the point of view of the clonal selection hypothesis, 

 it is rather irrelevant whether lymphocytes do or do not take 

 up relatively intact nucleotides from other cells and from 

 outworn lymphocytes. It would be of more interest to 

 examine the situation that arises when a homograft initiates 

 a rejection response, on the assumption that Medawar was 

 correct in regarding the effective antigen for homograft 

 immunity as being DNA protein. 



Our interpretation would be [a) that DNA nucleoprotein 

 is liberated wherever multiplication of cells is occurring and 

 that lymphocytes, particularly in germinal centres, may be 

 specially adapted to utilizing this material with minimal 

 breakdown for the construction of new cells as required. 

 [b) Foreign DNA-P acts like any other antigen except that, 

 for reasons not yet apparent, lymphocytes stimulated by 

 contact cannot (or only rarely can) take on the plasmacyte 

 form but produce only more lymphocytes, [c) Specifically 

 'labelled' lymphocytes in the immediate neighbourhood of 

 a homograft will, as soon as the foreign cells start dividing, 

 find plenty of the specific antigenic determinant and, 

 suffering damage, will liberate pharmacologically active 



112 



