THE FACTOR OF IMMUNIZATION 201 



In every version of clonal selection theory of immunity, it is 

 assumed, (i), that the capacity for immunological reactivity (for 

 convenience often referred to as antibody formation) is genetically 

 predetermined in the cell, and (2) that it is by virtue of this pre- 

 existing, though low-grade reactivity, that the antigen is recog- 

 nizable to the immunologically competent cell. In Burnet's views 

 on clonal selection, the whole population of immunologically 

 competent cells can be subdivided into clones, each of which has a 

 limited number of potentiahties for reaction to foreign antigens. 

 As the number of different antigens which any organism can be 

 confronted with is enormous, each clone is consequently supposed 

 to form an initially small fraction of the entire population. It is 

 only by the introduction of an antigen that the corresponding 

 clone will be selectively stimulated to proliferate and come to 

 form a substantial part of the entire population. 



In the words of Burnet and Burnet (i960) : ' ' antibody production 

 is presumed to occur in two phases corresponding (i) to the 

 inductive phase and (2) to the production phase of orthodox 

 immunological theories. In the induction phase two processes 

 occur; there is proliferation of the clone to produce many more 

 members, and the newly produced cells have an increased 

 reactivity with the corresponding antigenic determinant. In the 

 second phase, renewed contact with the antigenic determinant 

 results in proliferation, conversion to the plasma cell form and 

 production of antibody". 



What constitutes the exact mechanism of the proliferative 

 stimulus seems irrelevant in this context. The main points are 

 that proliferation is compelling if antibody production of the 

 clone is to be increased from its usually sub-detectable level to one 

 which is easily measurable (whether in terms of cellular or humoral 

 immunity), and that this proliferation brings the clone to occupy 

 a much higher proportion of the total population of immuno- 

 logically competent cells. 



Surely, in the CAM assay system, Burnet and co-workers 



