CHAPTER VI. 

 IMMUNITY. 



Studies in immunity have led to remarkably uniform results 

 in so far as the facts are concerned and there is great unanimity 

 in regard to the actual observations, both of the processes 

 which take place spontaneously in nature, as well as of those 

 which follow in the intentional experiments. There is, how- 

 ever, great difference of opinion upon the interpretation of these 

 phenomena, and several opposing theories have been advanced 

 in regard to the mechanism concerned, each theory finding very 

 eminent supporters. 



In view of these facts, it is necessary, in discussing immu- 

 nity, to give a definition covering its broadened application, to 

 cite the observations which have been recorded and to present 

 the prevalent explanations offered by the various authorities. 



It is scarcely necessary to go into a discussion of the various 

 theories which have been advanced from time to time, but which 

 have now been abandoned. 



Immunity, as formerly studied, embraced only considerations 

 of the insusceptibility of individuals or of races to an attack 

 of a given infectious disease. But the modern conception 

 is broader than this, and it is no bnger confined to immunity 

 proper, but extends to certain other processes which have been 

 found to bear a close resemblance, in certain respects, to im- 

 munity, and to be governed by laws very similar to those which 

 govern the latter. 



Immunity at present is made to include, besides insuscepti- 

 bility to infection — i. e., resistance to the invasion of living 

 bacteria — the processes concerned in the forming of the anti- 



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