486 BACTERINS, SERUMS — VACCINES — ANTITOXINS 



beast, are used today, after an interval of nearly fifty years, with 

 but very minor modifications. The next most important discoveries 

 were those of diphtheria antitoxin in 1890 by von Behring and 

 tetanus antitoxin in 1892 by von Behring and Kitasato. Wright and 

 Douglass, in 1903, further advanced the methods of vaccination for 

 therapeutic purposes by injecting dead bacteria into the bodies of 

 patients suffering from infections due to these particular organisms. 

 At first this method which is the basis of our modern " bacterin 

 therapy " was used only in chronic infections but now its application 

 has been extended to include the acute infections, notably typhoid in 

 man and with more or less success in dog distemper, hemorrhagic 

 septicemia and contagious abortion of cattle, etc. Serum of aniijials 

 immunized to the cause of a disease has been used for some time both 

 as therapeutic and prophylactic agents. The most notable examples 

 of the use of these agents are in cerebrospinal meningitis of man and 

 in hog cholera. The former was put on a firm basis by the experi- 

 mental studies of Flexner in 1905-6. The latter was discovered 

 and put into application by Dorset, ISTiles and McBride in 1908. 

 The far reaching success of the use of these agents cannot be ex- 

 pressed in exact figures. 



THEORIES OF IMMUNITY 



In order to understand the action of the products or agents to be 

 later described, a brief description of the theories of immunity is 

 necessary. Tor a full discussion of this subject the student or prac- 

 titioner is referred to the reference books given at the end of this 

 chapter. 



Immunity is the resistance to the invasion of a certain organism 

 or cause of disease which is possessed by an individual or animal. In 

 other words, it is the power an animal has to ward off or resist a 

 disease. Immunity is either natural or acquired. 



Natural Immunity. Certain species of animals do not, under 

 natural conditions, succumb to some diseases. For example, cattle 

 are not affected with glanders, dogs with anthrax or horses with black 

 leg. Natural immunity is the resistance certain species of animals 

 have to certain diseases, this resistance being bom with the indi- 

 vidual. Many theories have been devised to explain this phenomenon 

 but none of them are very satisfactory. 



Acquired Immunity is a resistance against disease brought 

 about by a variety of means, (a) Eecovery from a naturally con- 

 tracted attack of the disease, (b) By injecting the individual with 

 living but attenuated cultures or virus, (c) By injecting the indi- 

 vidual with fully virulent cultures or virus in sublethal amounts, 

 (d) By injecting dead bacteria and, bacterial extracts, (e) By in- 

 jecting bacterial products- (toxins), (f) By injecting the blood 



