92 Inside the Living Cell 



is one which develops very late in the development of the individual. 

 It was known that embryos do not have the ability to make anti- 

 bodies. Thus it appeared that all the antigens which are present in the 

 embryo when the immunological apparatus begins to function pro- 

 duce no response and only those antigens which are introduced after- 

 wards can make antibodies. In 1949 F. M. Burnet^ and F. Fenner fol- 

 lowed this idea up with the suggestion that, if foreign substances were 

 introduced into the embryo during the pre-immunological stage, they 

 would also be tolerated, i.e. they would not be treated as 'foreign' 

 antigens when the antibody machinery begins to function. 



This has been confirmed in laboratory experiments by Medawar 

 and his associates. They found that if cells from a strain cba of 

 brown mice are injected into a white of another strain a while the 

 latter are still in embryo, it will, when adult, accept skin grafts of 

 the former. This is a convincing demonstration that the antibody 

 mechanism does not respond to substances which have been present 

 in the embryo state. In fact, it is found that this state of 'tolerance' 

 also persists for a few days after birth. In the case of birds, it can be 

 produced by injecting the foreign material into the egg. Exactly how 

 the antibody-producing cells become unresponsive to the proteins 

 which were present in the embryo is still unknown and must wait for 

 further knowledge of the actual mechanism of production of anti- 

 bodies. 



There is another way — a rather drastic one — in which individuals 

 can be made unresponsive to foreign proteins and grafts of other 

 individuals. If the individual is subjected to a sub-lethal dose of 

 X-rays, the antibody mechanism is disorganized and for a time the 

 injection of foreign proteins will be tolerated, i.e. no antibodies will 

 be formed. This of course has its own dangers, because it leaves the 

 organism defenceless against bacteria, etc, and in fact, after receiving 

 a substantial dose of radiation, many animals die of infection. But it 

 also makes it possible to introduce foreign bodies such as red mar- 

 row cells, which, as we have seen, will help to keep the individual 

 going until it has repaired its own machinery for making red blood 

 cells. During the interval at which the animal is being helped by 

 foreign cells, it is what is known as a chimera, i.e. a composite of 

 cells of more than one genetic origin. 



Now Sir Macfarlane Burnet, om. 



