ANAMNESTIC RESPONSES 



reason for preferring the nuclear alternative and postulating 

 something equivalent to transduction rather than the transfer 

 of a self-replicating cytoplasmic entity, is that it is in line with 

 the current climate of opinion on such matters. Differentia- 

 tion is now regarded as being associated with nuclear changes, 

 and in bacteria, where induced changes and interactions 

 between organisms have been most adequately studied, it is 

 striking that with one doubtful exception all have been 

 shown to be of nuclear type. Transformation, recombina- 

 tion, transduction, lysogenesis, all represent modifications of 

 the central genetic control. The long persistence of immuno- 

 logical memory points in the same direction. And there is the 

 widespread intuition that where a cytoplasmic unit seems 

 to be self-replicating it possesses that privilege only while the 

 conditions for it are maintained appropriately by the nucleus. 



It has already been noted that the widespread sensitivity 

 of mesenchymal cells from tuberculous animals to P.P.D. 

 damage must represent the transfer of something from the 

 primary clones of cells. In this case we have the alternative 

 that a globulin with high affinity for the surface of other 

 mesenchymal cells may be liberated, and that we are con- 

 cerned not with a genetic phenomenon but merely with 

 a reaction between antigen and antibody at the surface of 

 a cell. This seems the more likely but the possibility of a wide- 

 spread genetic transduction cannot be excluded. 



If Medawar's (1957) opinion is correct that the effective 

 antigen in homograft immunity is a DNA nucleoprotein, we 

 must postulate that there is a constant traffic of such nucleo- 

 protein within the body. The breakdown of a tolerated homo- 

 graft, for instance, when a normal lymph node is implanted 

 in the tolerant host, must mean that an antigen from the 

 alien cells of the graft is constantly being set free. Directed 

 transduction by nucleoprotein liberated from replicating 

 cells might well play an important role in differentiation 

 and the various modifications of mesenchymal cell function 

 that are called for in health and disease. 



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