Protective Immunization. 29 



according to circumstances. As a definite cellular reaction in 

 such a case does not occur at all, or at best only in a minor 

 degree, and as the amount of protective antitoxic material intro- 

 duced is gradually used up, destroyed or excreted , (urine) and 

 no new antitoxine is afforded because of lack of proper reaction, 

 such passive immunity passes away after a short time (seven to 

 fourteen days). 



The recognition of this peculiarity has in practical application 

 led to , the coincident or successive employment of both methods 

 of immunization in case of certain of the infectious diseases. 

 Serum is first injected so as to induce a passive immunity in an 

 animal, and living germs are next inoculated so as to transfer a 

 passive into an active immunity ; the previous introduction of the 

 protective serum making the later inoculation with living microbes 

 practically free from danger. 



