Active and Passive Immunity 85 



ably associated with infection. The reactions of the body 

 toward bacteria in the infectious diseases are identical with 

 those toward other minute irritative bodies, and the reaction 

 toward bacterio-toxins is identical with those toward other 

 active substances, so that the only way by which a sat- 

 isfactory understanding of the phenomena of immunity 

 can be reached is by carefully comparing the reactions pro- 

 duced by bacteria and their products with those produced 

 by other active bodies. 



Immunity is called active when the animal protects itself 

 through its own activities, passive when its protection 

 depends upon defensive substances prepared by some other 

 animal and forced upon it. Thus, if a frog be injected with 

 anthrax bacilli, the leukocytes devour the bacteria, destroy 

 them, and so protect the frog from infection, the immunity 

 is active because it depends upon the activity of the frog's 

 phagocytes. But if a guinea-pig previously given anti- 

 tetanic serum be injected with tetanus toxin, and so recovers 

 from the toxin, the resisting power, conferred by the anti- 

 toxin previously injected, does not depend upon any activity 

 of the animal infected, which remains entirely passive. 



Immunity is largely relative. Fowls are immune against 

 tetanus, that is, they can endure, without injury, as much 

 toxin as tetanus bacilli can produce in their bodies, and 

 suffer no ill effects from inoculation. If, however, a large 

 quantity of tetanotoxin produced in a test-tube be intro- 

 duced into their bodies, they succumb to it. Mongooses 

 and hedgehogs are immune against the venoms of serpents 

 so as to be resistant to as much poison as is ordinarily in- 

 jected by the serpents, but by collecting the venom from 

 several serpents and injecting considerable quantities of it, 

 both animals can be killed. Rats cannot be killed by 

 injection with Bacillus diphtherias, and Cobbett* found 

 that they could endure from 1500 to 1800 times as much 

 diphtheria toxin as guinea-pigs, though more than this would 

 kill them. 



Carl Fraenkel has expressed the whole matter very 

 tritely when he says: ''A white rat is immune against 

 anthrax in doses sufficiently large to kill a rabbit, but not 

 necessarily against a dose sufficiently large to kill an ele- 

 phant." 



* " Brit. Med. Jour.," April 15, 1899. 



