THEORIES OF IMMUNITY 139 



centuries practical advantage has been taken of this observation in the 

 management of smallpox. In order to protect persons against a severe 

 attack of variola they were deliberately brought in contact with a 

 person suffering with a particularly mild form, in the hope that, by 

 inducing a mild attack of short duration, they would thus obtain pro- 

 tection against the severe, disfiguring, and fatal form of the disease. 



The practice, however, was not without danger to the individual and 

 to the public at large, as the induced disease would at times become 

 malignant, and constitute a focus of infection for an entire community, 

 When Edward Jenner discovered that inoculation with cowpox virus 

 could not produce smallpox, but would, nevertheless, stimulate the pro- 

 duction of specific antibodies and confer immunity against it, an enor- 

 mous forward stride was taken that has since proved a priceless boon in 

 helping to rid the world of the dreadful scourge of smallpox. 



The object of all these procedures has been to secure a resistance or 

 immunity to smallpox, either by inducing a mild form of the disease or 

 by protecting the individual by means of inoculation with a virus that 

 has been so changed in its passage through a cow as to render it unable 

 to produce smallpox, but yet is capable of stimulating the body-cells to 

 produce antibodies that will neutralize the effect of the true virus. This 

 induced resistance to a given infection constitutes immunity or resist- 

 ance, and since the body was purposely inoculated and the body-cells 

 rendered active in producing the antibodies, this form of resistance is 

 known as active acquired immunity. 



Many persons recover from an infection that may have been unusu- 

 ally severe not because the infecting agent became exhausted or died for 

 want of pabulum, but because it had been gradually worsted in the 

 battle with the defensive forces of the host. In many such instances 

 the host is now immune to this infection for a longer or shorter time, 

 because the body-cells have been so profoundly impressed that they con- 

 tinue generating defensive weapons or antibodies for some time after 

 the last vestige of the infecting agent has disappeared. Or, on the other 

 hand, the quantity of antibodies may be so great that they may persist 

 for varying periods of time, even for the remainder of life, ever on guard, 

 and ready to overwhelm their specific enemy should it ever again gain 

 access to the tissues. 



Here, then, arises the question concerning the mechanism of re- 

 covery from an infection, and since this is so intimately concerned with 

 the general subject of resistance to disease, it is considered under the 

 general head of immunity. 



