IMMUNITY. 53 



to apply the same treatment to persons bitten by rabid animals, with 

 results which tend to the belief that a real prophylactic for rabies 

 has been discovered. 



Immunity may also be produced by injecting the toxic products 

 existing in pure cultivations after removal of the bacilli. Salmon 

 was the first to produce immunity in this way, by utilising the toxic 

 products of the bacterium of hog-cholera, which were separated by 

 nitration from the living micro-organisms ; and shortly afterwards 

 Wooldridge demonstrated that filtered anthrax cultures contained 

 a substance which conferred immunity. Behring and Kitasato 

 produced immunity by mixing cultures with terchloride of iodine. 

 Yaillard filtered the cultures through porcelain, and attenuated the 

 products by heating at different temperatures. 



Lastly, in the course of Behring's and Kitasato's experiments, it 

 was found that the blood serum of animals rendered immune was 

 capable of conferring immunity on other animals. The injection 

 of the toxic products of pathogenic bacteria leads to the development 

 of substances in the blood to which the term " antitoxin " has been 

 applied. These protective substances neutralise or destroy the 

 injected poison, and blood serum which has thus been rendered 

 antitoxic can be utilised to confer immunity on other animals. 



Haffkine's system of vaccination as a protection against Asiatic 

 cholera is supposed to be based upon the principle of inducing the 

 formation of antitoxins or defensive proteids. 



MECHANISM OF IMMUNITY. 



E-aulin has shown that Aspergillus niger develops a substance 

 which is prejudicial to its own growth, in the absence of iron salts 

 in the nutrient soil, and Pasteur suggested that in rabies, side by 

 side with a living microbe, there is possibly some chemical product 

 or anti-microbe which has, as in Haulm's experiment, the power of 

 arresting the growth of the microbe. If we accept the theory of 

 arrest by some chemical product, we must -suppose that in the 

 acquired immunity afforded by one attack of an infectious disease 

 this chemical substance is secreted, and, remaining in the system, 

 opposes the onset of the micro-organism at a future time. In the 

 natural immunity of certain species and individuals we must suppose 

 that this chemical substance is normally present. 



Another theory is, that the micro-organisms assimilate the 

 elements which they require for their nutrition from the blood and 

 tissues, and render the soil impoverished or otherwise unsuitable for 



