ANIMAL ORGANISMS IN HEALTH AND DISEASE 519 



leucocytes are unable to cope with them, then enough tissue may be 

 broken down to cause the formation of a pus cavity, or abscess, and the 

 leucocytes become pus cells. 



552. Immunity. — Immunity may be defined briefly as the absence 

 of susceptibility to disease. It may be of three kinds: natural, acquired, 

 and artificial. 



Natural immunity is possessed by an animal because of the character 

 of its body. Many animals are naturally immune to certain diseases 

 to which others are susceptible. More or less immunity to some diseases 

 is possessed by certain human races; for example, the Jewish race is very 

 resistant to tuberculosis, while Negroes and the Irish are particularly 

 susceptible to it. There is also age immunity, adults being generally 

 free from so-called children's diseases. 



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. 



Artificial immunity 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 



