THEORIES OF IMMUNITY. 



state of the modified toxine would be the same. 

 Guinea-pigs were immunized by this modified 

 toxine, and instead of gradually increasing 

 doses, the amount used was always kept the 

 same. One set of animals received four doses 

 of the modified toxine, and another set eight 

 such doses. Of the serum of the first series of 

 animals 0.5 gram was required to protect against 

 a minimal lethal dose of the toxine — whilst of 

 the serum of the second series .005 gram was 

 sufficient to exert the same amount of protection. 

 In other words, twice the amount of toxine 

 gave rise to a serum of one hundred times the 

 strength. 



Of course many objections have been raised 

 against Ehrlich's theory, and after all is said 

 there are many points that are still obscure. 

 These objections are nowheres brought together 

 as in the papers by Ritchie (I. c), and the first 

 is this : How is it that after an animal has been 

 brought to a very high degree of immunity, 

 consisting according to Ehrlich's theory in the 

 presence of a large number of antitoxine units 

 in the blood, free and ready for combination, 



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