CLONAL SELECTION THEORY 



Direct template 



Indirect template 



A-^ 



Clonal selection 



Fig. 6. A diagram to suggest the essential differences between the three 

 theories. 



In the first (direct template theory) the antigen itself enters the cell 

 and stamps a complementary pattern on each globulin molecule after it 

 has been produced. 



In the indirect template theory the antigen, as it were, incorporates an 

 image of itself in the genetic mechanism of the cell. This is suggested by 

 putting a broken A in the nucleus — but the possibility of a cytoplasmic 

 mechanism of inheritance might appeal more to others. 



In the clonal selection theory, the image of the antigenic determinant 

 is present, shadowy perhaps, in the genetic mechanism before first con- 

 tact with antigen, which can stimulate, but not effectively enter, the cell. 

 The image and the globulin pattern can be sharpened up or modified by 

 mutation and selection. 



the most conspicuous of the normal antibodies, the iso- 

 agglutinins anti-^, anti-^ and anti-//(0). 



There are still conflicting opinions in regard to the 

 significance of the woagglutinins but, having regard to the 

 phenomena of immunological tolerance, that suggested by 

 Burnet and Fenner (1949) is still the most probable. They 



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