THE SECONDARY RESPONSE 



the Opinion that the primary response might not be a pro- 

 duction of antibody at all but a ' tooling-up' to allow response 

 to a secondary stimulus. A similar conclusion might be 

 drawn from the results of immunizing mice with bacterio- 

 phages used to construct Figure 4. 



If we attempt a more specific interpretation, it would take 

 the form that the commonest result of primary contact of an 

 antigenic determinant with a cell carrying potentially reactive 

 sites is to stimulate it to proliferative activity with the pro- 

 duction of a greatly increased number of cells of the clone, 

 but lymphocytic rather than of plasma cell type. The switch 

 to proliferation in plasma cell form takes place when these 

 activated lymphoid cells make a new contact with the same 

 antigen. Where antigen is being liberated from a depot in 

 the tissues over a period of days or weeks, the observed 

 response will be essentially secondary in type even if only 

 a single inoculum has been given. 



Further elaboration of this theme is better left till the 

 anamnestic reactions are being considered, but it may be 

 useful at this stage to draw attention to the interesting simi- 

 larity of this interpretation to that now current in regard to 

 the production of some adaptive enzymes by bacteria. 



In the case of y^-galactosidase of Escherichia coli, its induc- 

 tion by an appropriate inducer is a complex matter best 

 elucidated by the use of an inducer such as TMG (thio- 

 methylgalactoside) which is not a substrate of the enzyme 

 induced. The first phase of induction is the activation by 

 the inducer of a surface change in the bacterium, presumably 

 a result of the action of an induced enzyme (permease). 

 Once this action has taken place the inducer is in a position 

 to evoke or uncover the production of y^-galactosidase. The 

 important point relevant to the difference between primary 

 and secondary response to antigen is that once permease has 

 brought a bacterium to the inducible state, a culture geneti- 

 cally unchanged can be made to continue indefinite pro- 

 duction of y^-galactosidase in a medium containing a low 



71 



