112 DISCUSSION 



cell populations which are themselves not immunogenic and yet are 

 susceptible to the homograft reaction, is some tumour sub-lines which 

 we have established experimentally from a strain-specific sarcoma in 

 C57BL mice. These tumour sub-lines have acquired homotransplant- 

 ability: they cannot immunize the host, but they are extremely 

 susceptible to an immune response which is formed by the original 

 strain-specific tumour line or by other tissues of the same isoantigenic 

 constitution. However, in our case I am not quite sure whether one can 

 talk about a "barrier" like the one you have suggested. I shall discuss 

 these tumours in some detail in my paper. 



Barrett: Have you made any tests to see whether the epithelial 

 structures of the skin pouch are in themselves antigenic ? I would 

 suppose that they were, but I would think that question needed to be 

 answered. 



Billinghani: Our attempts to do this have so far been unsuccessful, 

 probably for purely technical reasons. That is why I laid great emphasis 

 on the "central implant experiment" with normal skin, to get round 

 that sort of objection. 



Barrett: This seems to me to be of extreme importance; I would think 

 when you hit the right technique it would be found to be antigenic. 



It seems to me that when you get further with your techniques that 

 this system offers a possibility to attack a problem that bothers us all. 

 Are we dealing here, not only with two types of response — the cell- 

 bound and the humoral — but are we also dealing with two different 

 classes of antigens : one the soluble type and one the particulate type 

 that is non-diffusible and would explain Dr. G. H. Algire's diffusion 

 chamber experiments and Prof Woodruff's experiments and perhaps 

 some of my own results ? It might be possible with your techniques to 

 show that these subepithelial barriers ipill pass certain well known sol- 

 uble diffusible antigens but will not pass such non-diffusible antigens as 

 are present in these epithelia. 



I might suggest that there is a biological advantage to the cheek 

 pouch of the hamster; Nature would be less than wise if she did not 

 provide a tissue with these special properties at a site where the animal 

 generally carries all sorts of antigenic food. 



Billiugham: If coarse sawdust is placed in the bottom of hamster cages 

 sharp splinters of wood not infrequently gain access to the cheek 



