THEORIES OF IMMUNITY. 



investigation, but in an immune serum the com- 

 plement is always present to a less degree than 

 the immune body. What the nature of these 

 bodies is is unknown, but the complements are 

 less resistant to heat than the immune bodies. 

 Further, the nature of the reaction which takes 

 place between bacteria, immune body, and com- 

 plement is disputed, and lastly, while the multi- 

 plicity of immune body is undoubted it is still 

 open to question whether there is a great number 

 of complements in each animal's body, or whether 

 there is, for each species at least, only one com- 

 plement which is capable of acting in conjunc- 

 tion with a great variety of immune bodies, so as 

 to produce a solvent effect on many different 

 kinds of bacteria. 



One of the greatest difficulties standing in the 

 way of the supporters of Ehrlich's theories is 

 the existence of the bactericidal property in the 

 blood serum of animals that have not been 

 treated in any way, and at the same time are 

 susceptible to the action of the bacterium that 

 their blood serum will destroy. The well-known 

 fact that the blood serum of the rabbit has a 



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