Acquired Immunity 91 



toxic products of that particular micro-organism, though experi- 

 ment may show the animal to be susceptible to it. Immunity 

 against any form of bacterio-toxin usually, though not necessarily, 

 determines that the micro-organism, though it may be able to invade 

 the body, can do very little harm. 



ACQUIRED IMMUNITY 



Acquired immunity is resistance against infection or intoxica- 

 tion 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 

 immunity, 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 sometimes also 

 acquire immunity through the parents. Thus in studying im- 

 munity of mice against ricin, Ehrlich found that the newly born 

 oSspring 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 duration. 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. — (a) Accidental Infection. — The most familiar form of 

 acquired immunity follows 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 possi- 

 ble 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 



