138 THE DIFFERENT TYPES OF IMMUNITY 



for any length of time after the infection has come to an end, if 

 indeed it has been increased at any time during its course, or there- 

 after. Evidently, then, still other influences must here be operative, 

 but what these influences are is still a matter of speculation. If we 

 bear in mind that a cell which has once been stimulated to active 

 antibody formation, probably responds to subsequent stimuli of 

 the same order with greater rapidity, and that those receptors no 

 doubt are regenerated in greatest number which have the greatest 

 affinity for the particular antigen to which they are "tuned," we 

 can imagine that the introduction of the corresponding organisms 

 at any period following the original infection or vaccination will 

 be successfully overcome in consequence of this specially active 

 response. 



This, however, is as yet a mere supposition, and the question 

 still remains unanswered why infection with certain organisms leads 

 to immunity and not with others. A discussion of the many possi- 

 bilities which present themselves in connection with this problem 

 would serve no useful purpose at this place. Much work still remains 

 to be done, but the main avenues along which profitable research 

 should be conducted are already clearly indicated. 



Mechanism of Antitoxic Immunity. While our knowledge of the 

 mechanism underlying the development of antibacterial immunity 

 is thus still very fragmentary, and really permits a clearer insight 

 into the manner in which infection can take place than into the 

 reasons why immunity may or may not develop, we have a much 

 better understanding of the modus operandi which forms the basis 

 of the antitoxic type of immunity. The organisms which are char- 

 acterized by a high degree of toxicity, such as the tetanus and the 

 diphtheria bacillus, as we have repeatedly pointed out, possess a 

 very low grade of infectiousness, so that they readily succumb to 

 the normal bactericidal agencies of the body. Their toxicity, how- 

 ever, is of such a high order that they are nevertheless formidable 

 pathogenic agents. It is accordingly surprising to find that some 

 animals are absolutely immune to the action of these toxins, and, 

 as a matter of principle, it is important to learn to what agencies 

 this remarkable natural immunity is due. In this connection 

 Ehrlich's side-chain theory regarding the origin and formation of 

 antibodies has been very helpful in arriving at a fairly definite 

 understanding. 



