THEORIES OF IMMUNITY. 



of experiments, in which he showed that animals 

 may be very susceptible to a very small number 

 of bacteria, whose blood-serum is exceedingly 

 bactericidal to a much larger number of the 

 same bacteria in the test-tube. For example, 

 the defibrinated blood and the blood serum of 

 the rabbit destroys numbers of the anthrax 

 bacilli, whilst the animals themselves succumb 

 also very promptly to the injection of a small 

 number of the bacteria in the blood vessels. 

 The assertion is made that such a contradiction 

 in results is only to be explained by the fact 

 that the blood itself undergoes a profound 

 change after its removal from the body. 



In 1891 the discovery by Behring and Kit- 

 asato of the existence of the antitoxines had not 

 made much impression so far as being con- 

 sidered a factor in the production of immunity 

 was concerned. It had been demonstrated as 

 occurring in the two diseases of diphtheria and 

 tetanus, and by Ehrlich for the vegetable alka- 

 loids — ricine, robine, and abrine; but it was 

 thought of as a special phenomenon peculiar to 

 these few diseases rather than as a general prin- 

 ciple capable of wide application. It was after 



25 



