THEORIES OF IMMUNITY. 



Ehrlich's theories, of the reactions by which 

 they are obtained. 



In the first group — those in which soluble 

 poisons are formed — the results are secured by 

 successful efforts to protect the tissues against 

 the soluble toxines, without reference to the 

 presence or absence of the bacilli themselves. 

 In the second the actual bacterial cells them- 

 selves must be considered as well as the toxic 

 substances that are very intimately connected 

 with them and only to be separated with diffi- 

 culty. 



Ehrlich's theories were first elaborated to 

 explain the conditions seen in the protection of 

 the tissues against the antitoxine of diphtheria 

 (passive immunity) , and were based upon the fun- 

 damental supposition that the action of the anti- 

 toxine upon the toxinewas of a chemical nature; 

 and this opinion was supported, among other 

 facts, by the demonstration that the two can be 

 titrated against each other " like an acid and an 

 alkali, and that it takes place more readily with 

 concentrated solutions than with weak.'' The 



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