Natural Immunity 89 



Immunity is called active when the animal protects itself through 

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

 sive substances prepared by some other animal entering into it. 

 Thus, if a frog be injected with anthrax bacilli, its leukocytes de- 

 vour the bacteria, destroy them, and so protect the frog from in- 

 fection; 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 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 diphtherise, 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 Frankel 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 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 infecting all kinds of animals; 

 indeed, it is doubtful whether any known organism possesses such 

 universally invasive powers. 



The micro-organisms of suppuration seem able to infect animals 

 of many different 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. 



