614 BACTERIOLOGY. 



of bacteria that in some way or other have been 

 modified. This modification may be artificially pro- 

 duced in the products 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 bac- 

 terial 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 antago- 

 nistic to the life of the invading organisms themselves, 

 but in most 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 inter- 

 pretation, it is "active" when resulting from an ordi- 

 nary non-fatal attack of infectious disease ; or from a 

 mild attack of infection purposely induced through the 

 use of living vaccines; or from the introduction oif 

 cultures of the bacteria that have been killed by heat ; 

 or from the gradual introduction of toxins into the tis- 

 sues until a marked antitoxic state is reached. It is 

 " passive " when occurring as a result of the direct trans- 

 ference of the perfected immunizing substance from an 

 immune to a susceptible animal, as by the injection of 

 blood-serum from the former into the latter. " Passive 

 immunity " is, in most cases, conferred at once, without 



