168 THE VARIOUS TYPES OF IMMUNITY 



the nature of the infecting parasite, and the presence or absence of 

 specific antibodies in the body-fluids. In many instances this type of 

 immunity is dependent upon non-specific causes is frequently relative 

 and seldom absolute. For example, fowls are immune to what may be 

 called an ordinary dose of tetanus toxin, but succumb readily to larger 

 doses; rats are highly immune to diphtheria toxin, and readily withstand 

 the effects of an amount equaling 1000 lethal doses for a guinea-pig, but 

 still larger doses may prove fatal; hedgehogs possess complete or almost 

 complete immunity for the amount of snake venom deposited in an 

 ordinary strike, but if the venoms of several snakes are collected and 

 injected at one time, the result is fatal. 



Species immunity is a type of natural immunity, best illustrated by 

 the immunity of man to certain diseases of the lower animals, such as 

 fowl cholera, swine-plague, distemper, Texas cattle fever, mouse septi- 

 cemia, etc.; and, conversely, by the immunity of animals to diseases 

 common to man, such as measles, cholera, typhoid fever, scarlet fever, 

 chicken-pox, etc. Although the close relation of man to the domestic 

 animals furnishes ample opportunity for infection, yet a complete 

 immunity is frequently observed. 



Racial immunity is that type of natural immunity existing among 

 members of the same species. For example, negroes are believed to 

 enjoy immunity to yellow fever and Mongolians to scarlet fever. As 

 a matter of fact, well-marked examples of racial immunity are extremely 

 rare, as not infrequently the disease in question may have been acquired 

 in early infancy in a clinically unrecognized form. 



Similarly, close biologic relationship is no guarantee that animals 

 will behave alike toward infection. For example, the white mouse is 

 immune to glanders, the house mouse is somewhat susceptible, and the 

 field mouse is highly susceptible. The rabbit, guinea-pig, and rat are 

 rodents, but though the rabbit and the guinea-pig are susceptible to 

 anthrax, the rat is largely immune. Mosquitos, though closely re- 

 lated, behave differently toward, the malarial parasite. The Culex 

 does not carry the parasite at all, and of the Anopheles, one species, 

 Anopheles maculipennis, is quite susceptible and well recognized as a 

 carrier of the parasite, whereas Anopheles punctipennis, though closely 

 related, is not susceptible to it. 



Examples of individual immunity, while not infrequent, are not con- 

 stant and seldom absolute. Certain persons appear to possess a defi- 

 nite immunity to scarlet fever and diphtheria, although they may be 

 freely exposed; others may pass through various epidemics of other 



