THE UNIQUENESS OF THE INDIVIDUAL 



away with altogether; or (d) the graft might be put in a position 

 in which, no matter what state of immunity might prevail, it 

 could not be got at by the cells that put the immunity into 

 effect. 



The first solution cannot be applied to homografts of the 

 kind I have been particularly concerned with, grafts which are 

 alive when they are grafted and which must remain alive if they 

 are to do their recipient any permanent good. Why this should 

 be so has already been explained: the antigenic make-up of a 

 graft is built into its genetic constitution. I therefore grieve at 

 the theoretically infirm attempts which have been made to 

 change the antigenic constitution of a graft by, for example, 

 growing it as a tissue culture in the body fluids of the animal 

 or human being on which it is ultimately to be transplanted — 

 accustoming it gradually (such is the feeble hope) to what it 

 will have to make do with later. No antigenic transformation 

 is in the least likely to occur under these conditions. Antigenic 

 transformations can occur under very special conditions which 

 have no bearing on the way in which homografts are used in 

 surgical practice; for example, if a graft consisting of isolated 

 tumour cells is transplanted to an animal which puts up a 

 certain feeble resistance to its growth, then the population of 

 tumour cells, considered as a whole, may change its antigenic 

 properties; but that I conceive to be due to a process of natural 

 selection, i.e. the selection, from a rapidly growing and prob- 

 ably variable population, of the particular variants that are 

 least antigenic to the host. 



Not all grafts are of the kind that need remain alive after 

 their transplantation; homografts of segments of blood-vessels, 

 for example, are building up an impressive record of successful 

 use in human surgery, but, to put it as a paradox, they are 

 successful as homografts because their failure does not matter. 

 What a vascular homograft does is to provide a fibrous tube 

 of the right shape and texture which, when its own cells die, 



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