Acquired Immunity 109 



invasion of pathogenic organisms, for neutralizing or destroy- 

 ing their toxins or 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 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 the mother's milk. 



Acquired immunity differs from natural immunity in 

 being less certain and of variable 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 intoxica- 

 tion, and either of these may be accidental or experimental. 



(A) Active Acquired Immunity. i. Immunity Ac- 

 quired 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 respec- 

 tive disease. Immunity thus acquired is not transmissible 

 to the offspring. 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 ob- 

 servable 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 the days when smallpox 

 was frequent, and has now become an extremely rare 

 disease. 



(b) Experimental Infection. i. Inoculation: The oldest 

 experiments in immunity date from unknown antiquity and 

 were practised in China and other Oriental countries for the 



