44 ACTIVE IMMUNIZATION 



3. The high degree and long duration of the immunity gained by pro- 

 phylactic inoculations. 



4. The possibility of immunization against pure parasites. 



5. The facility with which the inoculation material is preserved. 

 The disadvantages, however, may be summarized as follows: 



1. The manufacture of the inoculation material is rather complex and 

 with some pathogenic bacteria (pest), not without danger. 



2. The increased susceptibility during the interval between the inocu- 

 lation and the onset of immunity. 



The last point applies not only to aggressins, but equally to other 

 methods of active immunization. In times of an epidemic, aggressin 

 immunization should never be undertaken. 



When one bears in mind the great advantages derived from the employ- 

 ment of this form of immunization, its extensive use should be expected; 

 especially so as animal experimental work with the most important of in- 

 fectious bacteria: typhoid, cholera (Bail), colon (Salus), dysentery (Kiku- 

 chi), staphylococcus (Hoke), has proven it to be quite successful. It 

 is therefore no false prophecy, to say that this method will be em- 

 ployed more and more frequently in the future; particularly for pest, 

 results obtained in animal experimentation by Hueppe and Kikuchi have 

 more than sanctioned its employment in man. 



Other methods of immunization -based upon the Aggressin principles 

 have been advocated, but none have .attained any practical significance. 

 Mention however must, in passing, be made of the work of 

 Brieger's Brieger and his co-workers Mayer and Bassenge. Brieger 

 Bacterial had made extracts of typhoid and cholera bacilli, in the main 

 Extracts, identical with artificial aggressins. As far as his sterilization 

 was concerned, he obtained that by filtering the extract through 

 the Pukal filter. One should remember that by this procedure many 

 important substances are lost, but in spite of this, his results of inoculation 

 in man have been most encouraging, and there is a possibility that his 

 method may take the place of Wright's or Pfeiffer and Kolle's, as the reac- 

 tions are very much milder. 



Entirely different from the extracts of living bacteria are those made 

 from previously killed ones. Neisser and Shiga among others have immu- 

 nized against half parasites in this manner. This is not surprising since 

 the dead bacterial bodies can be similarly used for this purpose. As a 

 general rule, wherever dead bacterial bodies cannot be used for immuniza- 

 tion, their extracts will also be found inefficient. The oldest bacterial 

 extracts in use are the tuberculins. 



