DISCUSSION 109 



sitize to grafts elsewhere. The question of why anterior chamber grafts 

 are privileged thus became a little harder to answer. The explanation 

 we suggested is that they [a) produce sensitization slowly and (b) become 

 more difficult to dislodge as time goes on, and that their fate turns on 

 the balance of these processes. If the graft acquires a relatively high 

 degree of invulnerability before it has produced a high degree of im- 

 munity, then it survives. This is very different from your results. The 

 difference may be due to the fact that the anterior chamber lacks 

 lymphatic connexions, whereas the cheek pouch presumably has con- 

 nexions with the lymph nodes of the neck. 



While we are on this subject may I add that some people tend to 

 write about regional lymph nodes as if they were immutably the 

 lymph nodes of a particular site. Anybody who is concerned with the 

 surgery of cancer knows that this is not true, for if one lymphatic 

 channel gets blocked in some way, others open up and may carry 

 metastases to nodes which from the point of view of normal anatomy 

 would not be regarded as regional. 



One question — you said you could bring about rejection of your 

 grafts by adoptive immunization at any stage. We found that a thyroid 

 graft which had been in the anterior chamber for six months was not 

 disturbed by anything we then tried. More powerful weapons have 

 since been developed however and it might be worth trying again. In 

 your experiments what in fact was the longest interval between grafting 

 and adoptive immunization ? 



BiUiiigham: In our experiments with long-estabhshed cheek pouch 

 skin homografts no evidence has been forthcoming that they become 

 progressively less vulnerable to a state of sensitivity elicited in their 

 hosts by proxy. For example, a few milhon homologous leucocytes 

 injected into hamsters bearing cheek pouch homografts of 200 days' 

 standing will procure the rejection of these grafts within a week or two. 



The fmding that as few as i million leucocytes injected intravenously 

 will bring about destruction of these grafts has some bearing on the 

 question of whether the antigens in a homograft ever escape or are 

 released into its blood vessels and sensitize the animal that way. To try 

 and investigate this possibiHty a httle further, we took some hamsters 

 bearing long-established cheek pouch skin homografts and injected 

 about 20 milhon homologous bone marrow cells, from the same donor 



