Natural Immunity 95 



Immunity is called active when the animal protects itself through 

 its own activities, passive when the protection depends upon de- 

 fensive , substances prepared by some other animal entering into 

 it. Thus, if a frog be injected with anthrax bacilli, its leukocytes 

 devour the bacteria, destroy them, and so protect the frog from 

 infection; the immunity is active because it depends upon the ac- 

 tivity of the frog's phagocytes. But if a guinea-pig previously 

 given antitetanic serum be injected with tetanus toxin, and so re- 

 covers from the toxm, the resisting power, conferred by the antitoxin 

 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 in- 

 oculation. If, however, a large quantity of tetanotoxin produced 

 in a test-tube be introduced 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 infection 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 that would 

 kill them. 



Carl Frankel has expressed the whole matter very forcibly when 

 he says: "A white rat is immune against anthrax in doses suffi- 

 ciently large to kill a rabbit, but not necessarily against a dose suffi- 

 ciently large to kill an elephant." 



NATURAL IMMUNITY 



Natural immunity is the natural, inherited resistance against 

 infection or intoxication, peculiar to certain groups of animals, and 

 common to all the individuals of those groups. 



Few micro-organisms are capable of mfecting all kinds of animals; 

 indeed, it is doubtful whether any known organism possesses sUch 

 power. 



The micro-organisms of suppuration seem able to infect animals 

 of many difierent kinds, sometimes producing local lesions, some- 

 times invading rapidly with resulting bacteremia. The tubercle 

 bacillus is known to be pathogenic for mammals, birds, reptiles, 

 batrachians, and fishes, though it is still uncertain whether the 

 infecting organisms in these cases are identical or slightly differing 

 species. 



As a rule, however, the infectivity of bacteria and other micro- 

 organisms is restricted to certain groups of animals which usually 



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



