io6 Immunity 



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 reactions 

 toward bacterio-toxins are identical with those toward other 

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

 isfactory understanding of the phenomena can be reached 

 is by carefully comparing the reactions produced 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, 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 sufficiently immune against the venoms 

 of serpents to resist as much poison as is ordinarily injected 

 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 diphtheriae, 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 

 forcibly 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. 



