INFECTION AND IMMUNITY. 583 



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 of 

 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 

 the delay incidental to the usual modes of establishing 

 "active immunity." As a rule, "active" is more lasting 

 than "passive" immunity. 



8. That phagocytosis is effective in warding off dis- 

 ease in normal individuals only when the defenses of the 

 body are fully active ; when the number of invading 

 bacteria is relatively small or when the bacteria are pos- 

 sessed of low aggressive powers. It is probably a sec- 



