ANIMAL ORGANISMS IN HEALTH AND DISEASE 491 



Acquired immunity is the immunity which an animal enjoys by virtue 

 of having had a disease and having built up such a power of resistance as 

 makes it immune to succeeding attacks. To many infectious diseases 

 the human body develops resistance by the formation of antitoxins at 

 the time of the attack and by their continued formation afterward. Thus 

 conditions in the body are made unsuitable for the development of the 

 disease organisms should they again gain admission. This acquired 

 immunity may last for only a certain time or it may persist throughout life. 



In some cases individuals not naturally immune to a certain disease 

 may become so and may pass this immunity on to succeeding generations. 

 This may be termed inherited immunity. For instance, the child of a 

 mother who has had smallpox during the time when she was carrying the 

 child inherits the immunity acquired by the parent. 



Artificial iiyimunity is an immunity produced by artificial means; 

 there are several ways in which such immunity may be secured. (1) One 

 is by introducing into the body living but weakened cultures of the infec- 

 tive organism. A mild attack of the disease is produced which immunizes 

 the body against a serious attack, which would result from the entrance 

 of virulent organisms. An example of such an artificial immunity is that 

 resulting from vaccination for smallpox. The reaction to the vaccina- 

 tion is ordinarily not serious and results in immunity to the disease 

 itself. Immunity to rabies may also be produced in this way. (2) 

 Another method is by the introduction of virulent cultures in small doses 

 which the body can successfully withstand and as a result of which it 

 will build up an immunity to more serious infection. This mode of 

 securing artificial immunity has been practiced in the case of cholera and 

 bubonic plague. (3) Immunity against typhoid fever is secured by the 

 introduction into the body of extracts containing the dead bacteria. 

 Responding to the presence of these extracts, the body builds up the 

 appropriate antitoxin and thus safeguards itself against disease due to 

 the introduction of virulent organisms of the same kind. This is the 

 method now used in immunizing to plague and cholera. (4) Still another 

 way of securing artificial immunity is by the introduction of an antitoxin 

 developed in the body of another animal. The organism that causes 

 diphtheria in man, when grown in an artificial culture, will produce a 

 toxin. This may be introduced into the blood of a horse and the horse, 

 in response to its presence there, will manufacture an antitoxin. The 

 serum from the blood of the horse containing both the toxin and antitoxin 

 may then be injected into the body of a person and will not only confer 

 immunity but will tend to stop the disease if it has already been initiated. 

 The same type of procedure is followed in the case of scarlet fever, but in 

 tetanus the antitoxin alone is introduced. 



Many other toxins as well as infective organisms may be combated 

 in the body by the development of an appropriate antitoxin, which when 



