ACTIVE IMMUNITY 249 



of producing active immunity by the artificial introduction of 

 the organism is called vaccination, and a vaccine must there- 

 fore contain the organism. Vaccines for bacterial diseases are 

 frequently called bacterins. The use of the blood serum of 

 an immunized animal to confer passive immunity on a 

 second animal is properly called serum therapy, and the 

 serum so used is spoken of as an antiserum, though the latter 

 word is also used to denote any serum containing any kind 

 of an antibody (Chapters XXVII-XXXI). In a few 

 instances both the organism and an antiserum are used to 

 cause both active and passive immunity {serum-simulta- 

 neous method in immunizing against hog cholera) . 



In producing active immunity the organism may be intro- 

 duced (a) alive and virulent, but in very small doses, or in 

 combination with an immune serum, as just mentioned for 

 hog cholera. The introduction of the live virulent organism 

 alone is done only experimentally as yet, as it is obviously 

 too dangerous to do in practice, except under the strictest 

 control (introduction of a single tubercle bacillus, followed 

 by gradually increasing numbers— Barber and Webb). 

 More commonly the organisms are introduced (6) alive but 

 with their virulence reduced ("attenuated") in one of several 

 ways: (1) By passing the organism through another animal, 

 as is the case with small-pox vaccine derived from a calf or 

 heifer. This method was first introduced by Jenner in 

 1795 and was the first practical means of preventing disease 

 by vaccination. This word was used because material was 

 derived from a cow— Latin vacca. (2) By drying the organ- 

 ism, as is done in the preparation of the vaccine for the Pas- 

 teur treatment of rabies, where the spinal cords of rabbits are 

 dried for varying lengths of time— one to four days, Russian 

 method, one to three days, German method, longer in this 

 country. (It is probable that the passage of the "fixed 

 virus" through the rabbit is as important in this procedure 

 as the drying, since it is doubtful if the "fixed virus" is 

 pathogenic for man.) It would be more correct to speak of 

 this as a preventive vaccination agamst rabies, since the latter 

 is one of the few diseases which is not amenable to treatment. 

 The patient always dies if the disease develops. (3) The 



