IMMUNITY. 179 



tion, it must be evident, provided they have not fallen into error 

 somewhere, that they have succeeded in preparing antitoxin artifi- 

 cially, and if their statements be confirmed it must be admitted that 

 Ehrlich's theory concerning the nature of antitoxin must be consider- 

 ably modified. Final decision on this matter must await future in- 

 vestigations. 



When animals are immunized by successive treatments with a 

 microorganism or its products, the blood serum and other fluids ob- 

 tained from the body, acquire either bactericidal or antitoxic prop- 

 erties. In some instances the immunity secured is wholly anti-bac- 

 terial, while in otheis it is antitoxic. It will thus be seen that in the 

 production of artificial immunity we may expect to find marked dif- 

 ferences depending upon the microorganism used, and the kind of 

 animal immunized. We will first discuss bacterial immunity. The 

 bactericidal properties possessed by the fluids of the body of the im- 

 munized animal may manifest themselves only by an inhibitory action 

 on the growth of the germ, or by partially depriving it of its capa- 

 bility of elaborating toxins. Early in his investigations of this sub- 

 ject, Metschnikoff found that anthrax bacilli grown on the blood 

 serum of sheep immunized to this disease are without eflect upon 

 rabbits, but are still possessed of enough vitality to induce fetal 

 anthrax in mice. This indicates that there is something in the blood 

 serum of the immunized sheep which reduces the virulence of the 

 anthrax bacillus. In some instances the inhibitory action of the 

 immune serum manifests itself by depriving the bacterium of its 

 ability to produce certain of its characteristic products. Thus, as 

 has, already been stated, Charrin and Roger found that when the 

 bacillus pyocyaneus is grown in the blood serum of animals im- 

 munized to this microorganism it no longer produces its character- 

 istic coloring matter. The inhibitory action of the body fluids may 

 not be permanent, and when the bacillus is removed from their direct 

 influence it may recover all of its ordinary virulence. Bordet found 

 that when streptococci grown in the serum of immunized horses were 

 completely freed from this medium they are as virulent as those de- 

 veloped in the blood serum of an unvaccinated horse, and Roger ob- 

 served that the same microorganisms grow well in the serum of im- 

 munized rabbits, but that when thus grown and injected into other 

 animals along with some of the immune serum, they induce only a 

 temporary local disturbance. Next he inoculated rabbits with normal 

 streptococci placed in the blood serum of immunized animals and 

 found that the rabbits treated with this mixture remained well, while 

 those in which the streptococci and the serum were injected into dif- 

 ferent parts of the body succumbed to the infection. 



The phenomenon of agglutination is a manifestation of the in- 

 hibitory action of the sera of immunized animals on their homologous 

 bacteria. The fact that there may be immunity without agglutina- 



