THE UNIQUENESS OF THE INDIVIDUAL 



antigens liberated by the graft have a chance to act upon 

 lymphoid cells which are in normal working order; immunity is 

 built up and the homograft is destroyed. The peculiar import- 

 ance of this experiment is that it shows that the CBA skin 

 homograft, though a perfectly normal tissue, was manufacturing 

 antigens all the time — or rather, was manufacturing substances 

 that would have been antigenic if only the host had been 

 competent to recognize them for what they were. It is for this 

 reason that we suppose that the emission of nuclear particles 

 is a normal activity of living cells. The particles do nothing 

 recognizable, and cannot even be shown to exist, unless the 

 tissue from which they arise is transplanted to some other 

 individual as a homograft. The nuclear particles then act as 

 antigens and cause the tissue from which they originate to be 

 destroyed. Yet it would be idle to suppose that this nuclear 

 matter serves no function other than to make a nuisance of 

 itself in the entirely artificial act of grafting; it merely so 

 happens that grafting experiments of the design which I have 

 just described provide at present the only method by which 

 the nuclear matter discharged from cells can make its presence 

 known. No more need be said here of the possible functions in 

 normal life of the nuclear antigens which cause the homograft 

 reaction, for I discussed them at length in Section 3. 



The problem of how tolerance comes about is still unsolved. 

 For the present, the phenomenon of tolerance must be accepted 

 as one of the raw data of immunology, and no theory of the 

 mechanism of the immunological reaction wdll pass muster 

 unless it can explain the phenomenon of tolerance as well. 

 Haldane has suggested that an embryo has the power to 

 metabolize — to break down and make use or dispose of — 

 substances which the adult cannot metabolize; the adult makes 

 antibodies against them instead. If that is so, then tolerance is 

 the enforced retention of an embryonic modality of response; 

 and it would be in keeping with Burnet and Fenner's theory if 



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