IMMUNITY 67 



bacteria whose pathogenic power depends upon intracellular 

 poisons. Many kinds of anti-endotoxins or antibodies (a 

 term embracing antitoxins also but more commonly used as 

 here) are formed. The important ones are discussed under 

 the Actions of Bacterial Toxins and their Antibodies. 



Antitoxins. The production of antitoxins hinges on this 

 subject. Antitoxins may be described as the substances 

 produced in the blood or blood serum of animals injected with 

 the poisons elaborated by bacteria, but soluble and separable 

 from the germ cells. The toxins used to make the injections 

 into animals are obtained by growing the bacteria in broth 

 and then filtering oft' their bodies. They are distinguished 

 from the poisons described in a preceding paragraph in that 

 no destruction of the germs is necessary to produce these 

 separable toxins. 



Recapitulation. To recapitulate briefly, active acquired 

 immunity is produced by injection of living bacterial cells 

 when incapable of producing disease, or by their endocellular 

 poisons or extracellular separable toxins. In the case of the 

 latter it is possible to take from the blood serum of the 

 immune animals something which will neutralize the toxins 

 introduced, in other words, the principle involved in making 

 of diphtheria and tetanus antitoxin. The former, non-viru- 

 lent germs or their poisons, is used now as the basis of bacterin 

 treatment. 



Action of Bacterial Poisons and Their Antibodies. This 

 matter is very complicated and not by any means perfectly 

 understood by the most profound scientists. It is, moreover, 

 unnecessary to enlarge upon it in a work of this kind, and 

 tracing the methods as simply as possible is sufficient. 



The free soluble toxins stimulate the production of anti- 

 toxins which have an attraction for the toxin, and for it only. 

 Therefore when any free toxin and free antitoxin come 

 together they combine and one neutralizes the other. 



In the case of the reaction of bacterial cells or their endo- 

 toxins the result is more complicated. Many substances 



