DISCUSSION 403 



neonatal thymectomy, with a view to comparing the incidence of 

 tumours in such animals and their controls. If the thymus is producing 

 the originators of immunologically competent cells which play a part 

 in homograft reactivity against antigenically distinct clones of tumour 

 cells that arise spontaneously, then thymus regression, just before middle 

 age, might be correlated with the higher incidence of tumours that 

 occurs in the second half of life. 



Medawar: There has been some very recently pubHshed work on 

 the injection of antigens into the thymus itself Hasn't someone 

 demonstrated plasma cell transformation in the thymus as a result of 

 the local injection of antigen into the thymus ? 



Miller: Yes. These experiments were done by A. H. E. Marshall and 

 R. G. White (1961. Brit. J. exp. Path., 42, 379). If antigen is injected 

 intravenously there is no evidence of antibody formation inside the 

 thymus itself. If, however, the thymus is injured by cautery or if the 

 antigen is injected directly into the thymus, then there are histological 

 reactions suggesting that antibody formation actually takes place in the 

 adult thymus. It looks as if the thymus has some cells in it which are 

 already immunologically competent, but they normally don't produce 

 antibodies because the antigen doesn't get there. 



Mitchison: But of what organ is this not true ? If you inject antigens 

 directly into an organ, you usually observe formation of plasma cells in 

 it. 



Medawar: It depends upon in what numbers they are produced. The 

 interesting thing here is that the systemic injection of antigen doesn't 

 excite the thymus at all. 



Mitchison: It doesn't excite the skin either. But the skin is potentially 

 capable of producing antibodies, as Oakley has shown. 



Brent: Surely, in Oakley's experiments immunologically competent 

 cells were attracted to and trapped at the site of the antigen, and this 

 possibility can be ruled out here by the very fact that the thymus appears 

 to have a barrier. 



Medawar: If the thymus isn't on the pathway of lymphoid circulation, 

 as Gowans' experiments suggest, then this more obvious interpretation 

 can't be true. 



