THE UNIQUENESS OF THE INDIVIDUAL 



that they can no longer combine with the antibody that does 

 the harm. Mice which have been injected with extracts or 

 desiccates of their future donor''s tissues certainly do produce 

 orthodox serum antibodies in large amounts — antibodies 

 which, according to the reasoning of an earlier section, are not 

 the instruments of the homograft reaction; and we think it 

 possible that these antibodies have the power to combine with 

 or otherwise inactivate the antigens which cause transplanta- 

 tion immunity, perhaps as they issue from the grafts. 



The second '"specific"* method of interfering with the homo- 

 graft reaction is that which turns upon the principle of immun- 

 ological tolerance, and it deserves — or at all events is to receive 

 — a chapter to itself. 



5. IMMUNOLOGICAL TOLERANCE 



When antigens are injected into juvenile or adult animals, 

 they provoke some kind of immunological response. That is not 

 an empirical fact but a tautology: "antigens"* are so defined. It 

 is an empirical fact, however, that when antigens are injected 

 into embryos, or into newborns of the kind that are born very 

 immature, they do not elicit an immunological reaction. For 

 many years immunologists were content to dismiss this fact by 

 saying that the immunological faculty is one that develops and 

 matures like any other, and that embryos do not react upon 

 antigens simply because they are not yet sufficiently grown up. 

 This is a half truth; the other half of the truth is the subject 

 of the present chapter. 



In 1949, F. M. Burnet and Frank Fenner propounded a 

 theory of antibody formation which led them to make the 

 following prediction: that if an embryo were to be injected 

 with an antigenic substance, then, when it grew up, its power to 

 react against that antigen would be found to have been 

 seriously impaired. Almost all they had in the way of hard facts 



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