172 IMMUNITY. 



Pasteur ascertained that when the bacillus of chicken cholera is 

 grown on artificial culture media through successive generations, it 

 gradually loses its virulence and finally reaches a point when it is no 

 longer capable of inducing the disease in susceptible animals. Then 

 he found that animals treated with this non-virulent culture and sub- 

 sequently inoculated with virulent cultures remain unaffected. With 

 the anthrax bacillus it was found that there is no loss of virulence in 

 successive growths on artificial media. It was therefore necessary 

 to resort to other means of robbing this organism of its virulence. 

 Pasteur found that this could be accomplished in several ways, the 

 most important of which were : (1) Growth at relatively high tem- 

 peratures, and (2) growth in culture media containing small amounts 

 of certain antiseptics, such as carbolic acid. By both of these meth- 

 ods he was able to secure vaccines which were found to give tempo- 

 rary immunity. Vaccines of this kind are still employed, notably 

 for anthrax, but the immunity secured in this way is, in most in- 

 stances at least, of relatively short duration, and this method cannot 

 be said to be altogether successful. It is true that herds of cattle 

 and sheep may be protected from a prevailing epidemic in this way, 

 but revaccination must be frequently practiced, and there is always 

 danger, unless great care is exercised, of not sufficiently reducing the 

 virulence of the germ, and epidemics have been extended by the 

 employment of imperfectly prepared vaccines. 



Attempts to secure immunity by the employment of sterilized 

 cultures were probably suggested by the discovery of the fact that 

 immunity to the venom of snakes may be obtained by frequent suc- 

 cessive injections of non-fatal doses. As early as 1886, Sewall im- 

 munized pigeons to the venom of the rattlesnake, and soon thereafter 

 Salmon and Smith immunized animals to the bacillus of hog cholera 

 by successive treatments with sterilized cultures of the bacillus of 

 this disease. The later researches of Ehrlich with abrin and ricin 

 gave great impetus to study along these lines, and space cannot be 

 given here to even an enumeration of all the work that has been 

 done in the study of immunity by treatment with sterilized cultures. 

 In some instances the action of the toxin has been modified by heat- 

 ing or by the addition of certain chemicals, such as iodin and chlorin. 

 The production of immunity by the employment of sterilized cul- 

 tures or toxins is due in most instances, if not in all, to the formation 

 of antitoxins, and the explanation of the formation of these bodies 

 will be discussed later. 



A certain degree of immunity to some of the infectious diseases 

 may be secured by previous treatment of the animal with substances 

 of non-bacterial origin. The first hint that this might be accom- 

 plished was offered by Wooldridge, who reported that he had secured 

 immunity against anthrax by previous treatment of the animals with 

 extracts of testicles and thymus gland. Although subsequent inves- 



