Natural Immunity 107 



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 affecting all varieties 

 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, sometimes invading rapidly with resulting bactere- 

 mia. 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 have more or less resemblance to one another ; 

 thus, anthrax is essentially a disease of warm-blooded ani- 

 mals, though certain exceptions are observed, and Metschni- 

 koff has found that hippocampi (sea-horses), perch, crickets, 

 and certain mussels are susceptible. Among the warm- 

 blooded animals anthrax is most frequent among the her- 

 bivora, though some carnivora may also be infected. 



Close relationship is not, however, a guarantee that 

 animals will behave similarly toward infection. The rabbit, 

 guinea-pig, and the rat are rodents, but though the rabbit 

 and guinea-pig are susceptible to anthrax, the rat is immune. 

 This is still better exemplified in the susceptibility of mice 

 to glanders. The field-mouse seems to be the most suscep- 

 tible of all animals to infection with Bacillus mallei; the 

 house mouse is much less susceptible, and the white mouse 

 is immune. Mosquitos, though closely related, are differ- 

 ent in their immunity to the malarial parasite. The culex 

 does not harbor the parasite at all, and of the anopheles, 

 two very similar species seem to behave very differently, 

 Anopheles maculipennis being the common definitive host 

 of the parasite, while Anopheles punctipennis is not known 

 to be susceptible to it. The same differences may exist among 

 the members of the human species. It has been asserted 

 that Mongolians, and especially Japanese, are immune 



