28 MIXED IMMUNITY 



tion was a matter of great danger before the introduction of this 

 method. 



Passive immunity is also useful as a basis for active immunity 

 This will be described under the heading of Mixed Immunity. 



3. Passive immunity for a given bacterium or its products 

 cannot be made so potent as the active form for the same disease 

 in the same animal species. The reason is obvious : the passive 

 form only occurs in virtue of the presence in the blood of some of 

 the foreign serum, which can never form more than a fraction of 

 the whole fluid. The degree of the immunity may be sufficient for 

 all practical purposes, but can never reach the enormous height 

 met with in hypervaccinated animals. 



4. Active immunity we believe to be developed to some extent 

 in all, or almost all, infections, but the production of passive 

 immunity is impossible in very many cases e.g., tubercle (as far 

 as we know at present), infections with pyocyaneus, glanders, 

 malaria, and many other parasitic organisms. Perhaps in the 

 future we shall be able to procure active sera against all organisms, 

 but at present we have comparatively few of any value. 



MIXED IMMUNITY is a combination of the two forms already 

 described, in which the dangers and delay incidental to the induc- 

 tion of active immunity are avoided by the use of a protective 

 serum. It is really a succession of the two forms, the passive 

 immunity being developed at once as a consequence of the injec- 

 tion of the serum, whilst the active form develops later in conse- 

 quence of the vaccination. The process is sometimes called 

 sero- vaccination. 



It is not of great importance in human pathology, the chief 

 example being the form of typhoid inoculation suggested by 

 Besredka, and not yet used on a large scale. In it the killed 

 typhoid bacilli are submitted to the action of the immune serum, 

 from which they absorb certain protective substances and become 

 modified thereby. It is claimed that this treatment prevents the 

 development of the discomfort that follows the use of ordinary 

 typhoid vaccine, and that the immunity is developed very rapidly. 

 It may be followed by an injection of ordinary vaccine. 



The method is used to a considerable extent by veterinary 

 surgeons, and there are several modifications in the process, the 

 serum being injected either mixed with the virus, or before, or 

 after, or simultaneously in different sides of the body. Thus in 

 the treatment of South African horse-sickness the virus (the blood 



