ACTIVE IMMUNITY 231 



organism is called vaccination, and a vaccine must therefore 

 contain the organism. Vaccines for bacterial diseases are 

 frequently called bacterium. 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-simultaneous 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 (&) 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 smallpox vaccine derived from a calf or heifer. 



(2) By drying the organism, as is done in the preparation of 

 the vaccine for the Pasteur 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 impor- 

 tant in this procedure as the drying, since it is doubtful if the 

 "fixed virus" is pathogenic for man.) (3) The organism 

 may be attenuated by growing at a temperature above the 

 normal. This is the method used in preparing anthrax vac- 

 cine as done by Pasteur originally. (4) Instead of growing 

 at a higher temperature the culture may be heated in such 

 a way that it is not killed but merely weakened. Black-leg 

 vaccines are made by this method. (5) Chemicals are some- 

 times added to attenuate the organisms, as was formerly 

 done in the preparation of black-leg vaccine by Kruse's 



