CHAPTEE XIII. 



IMMUNITY. 



Immunity is an inherent insusceptibility to disease. 



It is a well-known fact that certain races and individuals 

 are not susceptible to some infectious diseases, and that others 

 are extremely susceptible to the same diseases. Negroes, as 

 a race, are immune to yellow fever. White persons born in 

 a yellow fever district, or who have lived in one for many 

 years, show a like resistance to the disease. Scarlet fever 

 and the other exanthems, except smallpox, are very common 

 in children, but are rarely seen in adults. One attack of 

 smallpox rarely is followed by a second ; but if so, the second 

 attack is a very mild one. Birds and snakes are not suscep- 

 tible to typhoid ; mice and rats, to diphtheria ; and pigs, to 

 snake-venom. 



The natural barriers to infection, such as the unbroken 

 skin and normal mucous membranes, can hardly be consid- 

 ered essential factors in the production of immunity. The 

 protection offered by immunity is solely against the bacterial 

 causes of disease which cannot develop in the body of the 

 immune. Or the living organism possesses the power of 

 diminishing the virulence of the germ and overcoming its 

 toxic effects. This might be termed tissue-endurance. The 

 immunity usually exists against both the germ and its toxin, 

 although not against an unlimited number or quantity of 

 either, and especially not of the latter. A susceptibility may 

 exist to the germ, but not to its toxin ; as, for instance, man is 

 susceptible to the action of the tubercle bacillus, but not to its 

 product, tuberculin. Immunity is always a relative condi- 

 tion. Fraenkel says that "a white rat is immune to anthrax 

 in amounts sufficiently large to kill a rabbit, but it is perhaps 

 not immune to a quantity sufficiently large to kill an ele- 

 phant. 77 



8 Bact. 113 



