52 MODERN BIOLOGIC THERAPEUSIS 



ciently established to produce disease. Since 

 the antibodies may be introduced in a short 

 space of time and in this way quickly induce 

 immunity, passive immunization for prophylac- 

 tic purposes is indicated when the danger of in- 

 fection is imminent and when it is impossible to 

 stimulate the individual's tissue-cells to produce 

 his own antibodies by active immunization with 

 a vaccine. Since the antibodies are produced in 

 another animal the serum when introduced into 

 the human body represents a foreign protein, 

 and the antibodies are retained for relatively 

 short periods of time because they are quickly 

 eliminated or destroyed. In active immuniza- 

 tion, however, the antibodies are in native sur- 

 roundings and the tissue-cells of the individual 

 continue to produce them for some time after 

 active stimulation has ceased thus, often in- 

 suring an immunity of long duration. Fre- 

 quently the two forms are used simultaneously, 

 as the immune serum will afford instant protec- 

 tion while the vaccine is stimulating the tissue- 

 cells to produce antibodies that will increase 

 and maintain the protection for a longer period 

 of time. This method has been used chiefly in 

 experimental and veterinary work. 



