20 BRENT, MEDAWAR AND RUSZKIEWICZ 



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DISCUSSION 



Silvers: I wonder if it would not be possible to differentiate the H-2 

 factors from the other factors by doing similar experiments with co- 

 isogenic resistant mice, where you would only be dealing with a single 

 H-2 difference. 



Medawar: Ideally, of course, one should use co-isogenic hnes. Un- 

 fortunately we suffer from a chronic shortage of mice and it would be a 

 grave logistical problem to change over from our domestic strains to 

 co-isogenic strains. 



Silvers: Because in the mouse there is apparently only one histo- 

 compatibility locus, the H-2, which is concerned with the production 

 of strong transplantation antigens, it might be advantageous to work 

 with other species. For example, we are maintaining two inbred strains 

 of rat which differ by at least 16 histocompatibility factors. Moreover, 

 four of these loci are concerned with the production of transplantation 

 antigens which can elicit as strong an immunological response as the 

 H-2 antigens of the mouse. By utilizing these rat strains it is conceiv- 

 able that one might be able to defme, in biochemical terms, the trans- 

 plantation antigens controlled by genes at more than one locus. This 

 would probably be difficult to accompHsh with the present methods in 

 the mouse. 



Medawar: This would indeed be worth studying. However, there 

 may be other strong histocompatibihty loci in mice which have not yet 

 been identified. 



Woodruff: On the question of the route of injection — this seems to 

 me very relevant to the differences in behaviour of organs transplanted 

 by vascular anastomosis and free grafts. There is presumably a com- 

 ponent of lymphatic and a component of vascular absorption in each 

 case, but the timing is different in the two. With organ grafts, local 

 irradiation may disturb the balance and perhaps reduce the lymphatic 

 components, partly by interfering with the regional lymph nodes and 

 partly by a direct strangulating effect on the lymphatic vessels. 



