ANTIGENS Z7 



although they will generally respond perfectly normally to other 

 antigens. This refractory state is called immunological tolerance. 



To explain it and related phenomena, modern theories of antibody 

 formation postulate, as was pointed out in Chapter 1, that the com- 

 bination of antigen with immature antibody-forming cells results in 

 the death of these cells or at least their elimination from the body. 

 It seems likely that the mechanism that produces acquired immunologi- 

 cal tolerance, whatever it is, accounts also for the failure of the body 

 to produce antibodies to its own circulating antigens. 



It is known that the rejection of tissue transplants from one in- 

 dividual to another, in contrast to the acceptance of transplants from 

 another part of the patient's body or from an identical twin, is an 

 immunological phenomenon. Acquired immunological tolerance has 

 been strikingly demonstrated in animals by injecting adult tissue 

 cells into embryos. Such injected embryos, when they are born and 

 grow up, may accept skin grafts, for example, from a donor of the 

 stock which provided the injected tissue, something they would not 

 do if not previously injected during fetal life. Billingham, Brent, and 

 Medawar (1953) believe that in such cases some of the injected cells 

 have survived in the recipients, thus accounting for the continued re- 

 ceptive state for transplants from that stock. 



A utoimmunisation 



Physiological mechanisms, like other machinery, can go wrong. It is 

 therefore not too surprising that occasional individuals are suspected 

 of producing antibodies to their own antigens. This process is called 

 autoimmunization, and naturally it is not a good thing when it occurs. 

 In fact, the process may be part of the etiology of a number of 

 hitherto mysterious diseases, most of them fortunately rare, such as 

 acquired hemolytic anemia, idiopathic thrombocytopenic purpura, 

 chronic leukopenia, periarteritis nodosa, lupus erythematosis, and 

 possibly other diseases. 



The production of autoantibodies is made possible by a number 

 of abnormal factors, which may include (a) modification of one of 

 the patient's own antigens by combination with a drug, a bacterial 

 toxin, or something of the sort, so as to make it at least partially 

 "foreign" to his circulation, and (b) an unusual propensity of the 

 patient to form antibodies in general. 



