578 BACTERIOLOGY. 



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 modi- 

 fied 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 the living leuco- 

 cytes contribute to the circulating fluids certain sub- 

 stances that are important to the establishment of im- 

 munity. 



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 iu these tissues of antitoxic and other 

 anti-bodies, which circulate free in the blood, and in a 

 variety of ways serve to protect the tissues from the 

 harmful efl^ect of extraneous intoxicants and irritants, 

 in some cases acting principally as antidotes to a toxin, 

 in others exhibiting more the germicidal (bacteriolytic) 

 than the antitoxic property. 



