THE UNIQUENESS OF THE INDIVIDUAL 



something as near to permanence as mortality allows, and this 

 variation is an expression, perhaps the completest single 

 expression, of the genetical relationship between the donor 

 and the host. 



A logical, or more properly a chronological, analysis of the 

 reaction that leads to the rejection of homografts should begin 

 with the antigens, the substances that cause immunity; proceed 

 from the antigens to a consideration of whereabouts in the host, 

 and how the reaction against them begins to take effect; and 

 end \^ath an analysis of the effector mechanisms of immunity, 

 i.e. of how the reaction of the host is, as civil servants say, 

 actually *'implemented\ To avoid too many digressions, I pro- 

 pose, however, to begin with the hosfs response and to leave 

 any consideration of the antigens until the end. 



There is one thing that must be said immediately about the 

 response of the recipient, for otherwise one or two of the 

 experiments I shall describe below will be unintelligible. It is 

 only when a human being or other animal is confronted with a 

 homograft/br the first time that the homograft enjoys a latent 

 period during which it behaves like a graft of the recipient''s 

 own skin. A second graft from the same donor, transplanted 

 after the rejection of a first, is set upon almost immediately; it 

 does not heal properly, it never acquires a working vasculature, 

 and it never even begins to reorganize itself internally or to 

 develop new skin glands or hair. From a surgical point of view 

 its destruction is virtually instantaneous, though its epithelial 

 cells can survive a few days until they die of inanition. If, 

 however, the second graft comes from a donor genetically 

 different from the first, its behaviour may be almost completely 

 normal. That is what one would expect, unless the donors of 

 the first and second grafts happen to be closely related, in 

 which case the second graft is summarily destroyed. This 

 behaviour strengthens the analogy between an animaPs re- 

 action against homografts and its reaction against a disease. 



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