DISCUSSION 113 



pouches and penetrate their walls, where they become embedded in 

 their connective tissue. On dissecting out pouches for grafting pur- 

 poses we have frequently observed these foreign bodies. Rather 

 surprisingly there appears to have been very little local reaction to their 

 presence. We have never seen an inflammatory or granulomatous re- 

 action of the type that would certainly be encountered in the normal 

 integument of the hamster or other mammals. 



Barrett: How deep can these splinters go ? 



Billingharn: They may go down to the level of the compact fibrous 

 connective tissue of the cheek pouch. 



Woodruff: But how does it reject the bacteria which are sure to enter 

 the tissues along with the splinter ? This absence of immunological 

 response isn't always advantageous. 



Brent: Isn't the immunological reaction itself very often harmful; the 

 harm need not always be done by the presence of the antigen or the 

 bacteria per se, need it ? 



Woodruff: There is a tendency to look at it that way, but I think it 

 would be going much too far to suggest that the capacity for immuno- 

 logical reaction is of no value for survival. 



Billwghani: I might add that several people with whom we have been 

 in correspondence have told us that the skin of the hamster cheek pouch 

 is much more resistant to carcinogens than the skin of the general 

 integument. 



Hildemmui: I wonder if a direct test of Dr. Billingham's hypothesis 

 could not be made by pre-injecting the donor with tritiated thymidine, 

 and assuming the mitotic cells of cheek pouch skin would readily take 

 up the label, make the usual homograft and subsequently determine the 

 extent of DN A~labelling of the recipient's lymph nodes. This could 

 provide direct evidence of whether graft cells were getting into the 

 host. 



Russell: In experiments which we have not published, using tritiated 

 thymidinc-labelled skin grafts in mice, both isogenic and allogeneic, a 

 good deal of labelled DNA can be found soon after grafting in the 

 regional lymph nodes near both types of grafts. There may be a little 

 more rapid release of labelled material from allogeneic than from iso- 

 genic grafts, although we aren't very sure of this. Nevertheless it is 

 clear, even in the isogenic combination, that there is a lot of release of 



