482 BACTERIOLOGY. 



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

 either "active" or ** passive." According to this in- 

 terpretation, it is ' 'active" when resulting from an or- 

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

 mild attack of infection purposely induced through the 

 use of living vaccines ; or from the gradual introduc- 

 tion of toxins into the tissues until a conspicuous stage 

 of tolerance is reached. It is ''passive" when occur- 

 ring as a result of the direct transference of the per- 

 fected 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, though frequently observed, is 

 not essential to the establishment of immunity, but is 

 more probably a secondary process, the bacteria being 

 taken up by the leucocytes only after having been mod- 

 ified in virulence through the normal germicidal activity 

 of the serum of the blood and of other fluids in the 

 body. It is, however, probable that important factors 

 in the establishment of immunity are substances secreted 

 and thrown into the circulating fluids by the living leu- 

 cocytes. 



9. That, of the hypotheses advanced in explanation 

 of acquired immunity, the one worthy of greatest con- 

 fidence is that which assumes immunity to be due to 

 reactive changes on the part of the tissues that result 

 in the formation in these tissues of antitoxic substances 

 capable of neutralizing the poisons produced by the 

 bacteria against which tlie animal has been immunized; 



