INFECTION AND IMMUNITY. 615 



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 

 effective in warding off disease in normal individuals 

 only when the normal defenses of the body are fully 

 active ; when the number of invading bacteria is rela- 

 tively small ; or when the bacteria are possessed of low 

 aggressive powers ; while in acquired immmunity it is 

 more probably a secondary process, the bacteria being 

 taken up by the leucocytes only after having been 

 modified in virulence through the 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 in 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 effect 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. 



