232 IMMUNITY 



method in Germany. This method is no' longer used to any 

 great extent. (6) Within the past few years the workers in 

 the Pasteur Institute in Paris have been experimenting with 

 vaccines prepared by treating living virulent bacteria with 

 antisera ("sensitizing them") so that they are no longer 

 capable of causing the disease when introduced, but do 

 cause the production of an active immunity. The method 

 has been used with typhoid fever bacilli in man and seems 

 to be successful. It remains to be tried out further before 

 its worth is demonstrated (the procedure is more compli- 

 cated and the chance for infection apparently much greater 

 than by the use of killed cultures). (7) Growing on artifi- 

 cial culture media reduces the virulence of most organisms 

 after a longer or shorter time. This method has been tried 

 with many organisms in the laboratory, but is not now used 

 in practice. The difficulties are that the attenuation is very 

 uncertain and that the organisms tend to regain their 

 virulence when introduced into the body. 



In producing active immunity against many bacterial 

 diseases the organisms are introduced (c) dead. They are 

 killed by heat or by chemicals, or by using both methods 

 (Chapter XXX). 



When the products of an organism are introduced the 

 resulting immunity is against the products only and not 

 against the organism. If the organism itself is introduced 

 there results an immunity against it and in some cases also 

 against the products, though the latter does not necessarily 

 follow. Hence the immunity may be antibacterial or anti- 

 toxic or both. 



Investigation as to the causes of immunity and the various 

 methods by which it is produced has not resulted in the dis- 

 covery of specific methods of treatment for as many dis- 

 eases as was hoped for at one time. Just at present progress 

 in serum therapy appears to be at a standstill, though vac- 

 cines are giving good results in many instances not believed 

 possible a few years ago. As a consequence workers in all 

 parts of the world are giving more and more attention to 

 the search for specific chemical substances, which will destroy 

 invading parasites and not injure the host {Chemotherapy). 



