THE UNIQUENESS OF THE INDIVIDUAL 



is repopulated by cells arising from its recipient; a new lining 

 of endothelial cells is laid down on the inner surface of the 

 graft, and cells of the connective tissue family penetrate the 

 interstices of its fibrous skeleton and convert it into a plausible 

 and efficient imitation of a normal blood vessel. That vascular 

 homografts, as living entities, '"die'' and are none the worse for 

 it is shown by the fact that they need not be alive even to 

 begin with. Vascular homografts which have been killed 

 (though kept in a lifelike condition) by drying from the frozen 

 state or by prolonged storage at very low temperatures seem 

 to do as well as living grafts; synthetic plastic tubes will also 

 serve. Vascular homografts die, but they enjoy the privilege 

 of reincarnation. 



The second and third ways of getting round the homograft 

 reaction are complementary to each other; either to transplant 

 a homograft into a position in which the antigens manufactured 

 by it never reach a centre of response, so that the host is never 

 officially aware of its existence, or to transplant it into a 

 position in which the effectors of the immunological response 

 are unable to get at it. The main seat of the hosfs reaction 

 against homografts, I have explained, is the regional lymph 

 nodes, and antigens reach them through the regional lymph- 

 atics. The brain has no lymphatic drainage in the ordinary 

 sense, nor is it monitored by lymph nodes. It is therefore 

 entirely intelligible that homografts transplanted into the 

 substance of the brain should fail (as they do indeed fail) to 

 elicit an immunological reaction. 



The complementary case is best exemplified by the cornea. 

 Homografts transplanted into the cornea are in a kind of 

 sanctuary. Their condition is to be likened, perhaps, to that of 

 homografts transplanted into the little plastic bags, permeable 

 to fluids but not to cells, which I mentioned in an earlier sec- 

 tion. The cornea is a non-vascular structure, so that the cells 

 which are the effectors of the hosfs reaction against homo- 



170 



