Acquired Immunity 97 



ACQUIRED IMMUNITY 



Acquired immunity is resistance against infection or intoxication 

 possessed by certain animals, of a naturally susceptible kind, in 

 consequence of conditions peculiar to them as individuals. It is a 

 peculiarity of the individual, not of his kind, and signifies a subtile 

 change in physiology by which latent defensive powers are stimulated 

 to action. The reactions in general correspond with those of 

 natural immunityj and comprise mechanisms for overcoming the 

 invasion of pathogenic organisms, for neutralizing or destroying 

 their toxins or for both. As an acquired character and an individual 

 peculiarity it is not transmitted to the offspring, though these some- 

 times also acquire immunity through the parents. Thus in study- 

 ing immunity of mice against ricin, Ehrlich found that the newly 

 born offspring of an immune mother were not immune, though they 

 subsequently became so through her milk. 



Acquired immunity differs from natural immunity in being more 

 variable in degree and diuration. The animal may be immune 

 to-day, but lose all power of defending itself a month hence. 



Natural immunity is always active, but certain forms of acquired 

 immunity are passive. 



Immunity may be acquired through infection or intoxication, and 

 in either case may be accidental or experimental. 



(A) Active Acquired Immunity. — i. Immunity Acquired through 

 Infection. — (o) Accidental Infection. — The most familiar form of 

 acquired immunity foUows an attack of an infectious disease. Every 

 one knows that an attack of measles, scarlatina, varicella, variola, 

 yellow fever, typhoid fever, and other common infectious maladies, 

 is a fairly good guarantee of future exemption from the respective 

 disease. Immunity thus acquired is not transmissible to the off- 

 spring. Almost everybody has had measles, yet almost all children 

 are born susceptible to it. It is not necessarily permanent, as is 

 shown by the not infrequent cases in which second attacks of measles 

 occur. In some cases, as after typhoid fever, the immunity is not 

 at first observable and the patient may suffer from relapses. Later 

 it becomes well-established and no repetition of the disease is possible 

 for years. 



Sometimes the infection, by which immunity is acquired, is not 

 exactly similar to the disease against which it affords protection, as 

 in the case of vaccinia, which protects against variola. It is still 

 controversial, however, whether cow-pox is variola of the cow or an 

 entirely different disease. Cow-pox was, however, common in 

 days the when smallpox was frequent, and has now become 

 extremely rare. 



(6) Experimental Infection. — i. Inoculation: This is an attempt 



to prevent the occurrence of a fatal attack of an infectious disease, 



by inducing a mild attack of the same disease when the individual 



is in good health, and at his maximum resisting power. The oldest 



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