THE UNIQUENESS OF THE INDIVIDUAL 



to devise methods of overcoming the homograft reaction by 

 techniques of desensitization, many unwary workers have been 

 guided by the quite unfounded behef that the several tissues of 

 the body contain ''homograft antigens'* pecuhar to themselves. 

 To make a skin homograft survive, therefore, it was thought 

 necessary to inject its intended recipient with some desensit- 

 izing preparation made from skin itself. The basis of this judge- 

 ment can now be seen to be quite illusory. If ever it becomes 

 possible to desensitize an adult animal for the purpose of 

 executing a homograft, it will doubtless be done by injecting 

 it beforehand with some preparation of its future donor''s 

 tissues; but the tissue used for that purpose need not be of the 

 same kind as that which is ultimately to be grafted; blood 

 leucocytes should serve the purpose as well as any other kind 

 of living cell — a most important dispensation, because blood 

 is of all tissues the easiest to come by and the easiest to spare. 

 The second property of tolerance I wish to mention also 

 bears directly upon the nature of the antigens that cause the 

 homograft reaction. A state of tolerance, no matter how long 

 it has prevailed, can be brought to an end simply by re- 

 equipping the tolerant animal with normal, and therefore 

 immunologically competent, lymph node cells. Consider an 

 A-line mouse which has been made to accept a homograft of 

 skin from a mouse of strain CBA, and let it be supposed that 

 the homograft, bearing its characteristic coat of brown fur, 

 has long been fully accepted by its host. The homograft can 

 be destroyed, and the state of tolerance brought permanently 

 to an end, simply by injecting the A-line mouse with normal 

 lymph node cells taken from normal A-line donors. Until then, 

 the antigens given forth by the CBA skin homograft were 

 unable to elicit a reaction, because its recipients lymph node 

 cells had been incapacitated by the CBA cells to which they 

 were exposed in embryonic life. But when the A-line host has 

 been refurnished with normal lymph node cells, then the 



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