292 BACTERIOLOGY 



3. That in the serum of the normal circulating blood of 

 many animals there exists a substance that is capable, out- 

 side of the body, of rendering inert certain pathogenic 

 bacteria, but which is, however, present in such small 

 quantities as to be ineffective, either for the protection of 

 the animal or for the cure of infection when introduced into 

 the body of another animal already infected. 



4. That immunity is most frequently seen to follow the 

 introduction into the body of the products of growth of 

 bacteria that in one way or another have been modified. 

 This modification may be artificially produced in the prod- 

 ucts of virulent organisms, and then introduced into the 

 tissues of the animal; or the virulent bacteria may be so 

 treated that they are no longer of full virulence, and when 

 introduced into the body of the animal will produce poisons 

 of a much less vigorous nature than would otherwise be 

 the case. 



5. That immunity following the introduction of bacterial 

 products into the tissues is apparently due to the formation 

 in the tissues of another body or other bodies that act as 

 antidotes to the poisons, and thereby protect the tissues 

 from their hurtful effects. 



6. That this protecting proteid which is generated by the 

 cells of the tissues need not of necessity be antagonistic to 

 the life of the invading organisms themselves, but in some 

 cases must be looked upon more as an antidote to their 

 poisonous products. 



7. That immunity, as conceived by Ehrlich, may be 

 either "active" or "passive." According to this interpre- 

 tation, it is "active" when resulting from an ordinary non- 

 fatal attack of infectious disease; or from a mild attack 

 of infection purposely induced through the use of living 



