THEORIES OF IMMUNITY. 



is evidently capable of indefinite expansion, as 

 well as of being applied to all the reactions at 

 present known to occur. 



Ehrlich first conceived his theory, or at least 

 the beginning of it, to explain the occurrences 

 in passive immunity; specifically, those seen in 

 the application of the antitoxine treatment of 

 diphtheria. 



For the proper understanding of the case the 

 two types of bacterial action must be borne in 

 mind. The first, of which diphtheria is an 

 example, is that in which the results seen follow 

 the action of soluble poisons; which are de- 

 veloped during the growth of the bacteria; 

 which pass away from the site in which the 

 development of the bacteria may be going on; 

 and which produce their results at a distance 

 from that site. Diphtheria and tetanus are both 

 examples of this form of bacterial action; in both 

 of these diseases the bacteria producing them 

 grow mainly in one place in the body, and pro- 

 duce poisonous substances that act at a dis- 

 tance from this place. In the cases mentioned, 

 the poisonous substances also act upon special 



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