IMMl'NITY 1247 



are used. In the case of human vaccination, "killed" cul- 

 tures are usually employed, because of the possibility that 

 the virulence of the weakened organism may accidentally be 

 regained. The organisms are killed in such a way as to de- 

 stroy their reproductive power, but not to change them 

 chemically to such an extent that when introduced into the 

 body they will not stimulate the cells to the production of 

 the anti-bodies. The manner in which the different vac- 

 cines are made will be discussed in the treatment of the 

 specific diseases. Vaccines are used not only to prevent but 

 also to cure disease. 



In the production of passive immunity the anti-bodies 

 are transferred from the body of the animal in which they 

 have been actively formed to the animal to be protected. 

 This transfer is accomplished by withdrawing a portion 

 of the blood from the immune animal and injecting it into 

 the animal that it is sought to protect. Since the blood 

 serum is used, the term protective serum or anti-serum is 

 often used. In many instances the anti-serum contains 

 primarily a substance that neutralizes the toxin. The term 

 antitoxin is therefore often used. 



The blood of a hog that has recovered from hog cholera 

 will contain sufficient anti-bodies to protect the individual 

 against a subsequent attack, but not a sufficient amount 

 so that the blood would bestow any marked degree of protec- 

 tion on another animal when inoculated w r ith an amount 

 that would be practicable to use. In order to make the 

 method of practical value, the immune animal is forced to 

 manufacture a larger amount of the anti-bodies than would 

 normally be produced. An animal so treated is said to be 

 hyper-immunized. In preparing hog-cholera serum, this 

 is accomplished by injecting into the body of the immune 

 hog a large quantity of blood from a hog that is already 

 sick with hog cholera. The specific organism causing hog 



