THE DEFENSES OF THE BODY 311 



vaccines; or from the introduction of cultures of the bacteria 

 that have been killed by heat; or from the gradual intro- 

 duction of toxins into the tissues until a marked antitoxic 

 state is reached. It is "passive" when occurring as a result 

 of the direct transference of the perfected immunizing sub- 

 stance 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 establish- 

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

 lasting than "passive" immunity. 



8. That phagocytosis is effective in warding off disease 

 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 possessed of low 

 aggressive powers. It is probably a secondary process, the 

 bacteria being taken up by the leukocytes only after having 

 been modified through the opsonizing activity of the serum 

 of the blood and of other fluids in the body. 



9. That of the hypotheses advanced in explanation of 

 acquired immunity, the one worthy of greatest confidence 

 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 antibodies, which 

 circulate free in the blood, and in a variety of ways serve 

 to screen the tissues from the harmful effect of extraneous 

 intoxicants and irritants, in some cases acting principally 

 as antidotes to toxins, in others exhibiting more the 

 germicidal (bacteriolytic) than the antitoxic property. 



