INTRODUCTORY AND GENERAL ig 



5. In some cases (as has been noted above) passage through 

 animals diminishes the virulence. In some cases this can be 

 exalted by passage through a series of animals of one species and 

 diminished by the use of another. Pasteur showed this to be the 

 case in swine erysipelas, the potency of which (as tested on pigs) 

 is increased by passage through pigeons and decreased by passage 

 through rabbits. Cultures thus attenuated are used as vaccines. 



The term ACQUIRED IMMUNITY is one that is used to denote an 

 increased resistance to an organism dependent on some modifica- 

 tion in the animal's constitution due to some definite process to 

 which it is subjected, but not including the modifications due to 

 improvements in the general health due to betterment of the 

 environment. For example, a person living in insanitary sur- 

 roundings will undoubtedly acquire a higher degree of resistance 

 to the tubercle bacillus on being moved to more healthy ones, but 

 we do not speak of that as acquired immunity. The distinction 

 is this : The elevation of the natural resisting powers due to 

 improvement in the general vitality is a more or less general one, 

 and affects the immunity to most or all bacteria almost equally ; 

 whereas in acquired immunity in the narrower sense, to which 

 the use of the term is restricted by pathologists, the alteration is 

 in the powers of resistance to one bacterium only. For example, 

 a debilitated person removed to a more healthy environment, 

 given better food, tonics, etc., would become more resistant to 

 the attacks of smallpox, and to other diseases as well ; we should 

 speak of that as an augmentation of the natural immunity. But 

 after an attack of smallpox, or after vaccination, his immunity 

 to smallpox is enormously increased, whereas his resistance to 

 other organisms is unaltered ; this is acquired immunity. 



This is expressed by the use of the word specific, embodying an 

 idea difficult to define, but implying a direct relationship of cause 

 and effect, and, moreover, that a certain effect is only produced 

 by a certain definite cause. Thus the toxin of diphtheria is 

 specific for the diphtheria bacillus in the sense that it is produced 

 by that organism, and by no other ; diphtheria antitoxin is specific 

 for diphtheria toxin, since it is produced only as a result of the 

 injection of that substance ; the reaction caused by the injection 

 of tuberculin into a tuberculous animal is specific, etc. 



In most instances there is another difference between a rise in 

 natural immunity and the development of acquired immunity, in 

 that the latter is much stronger. Thus, the power of resistance 



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