DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 197 



reached broad and beneficial therapeutic application, and innumerable 

 lives have been saved by their use. 



Passive immunization against, microorganisms not char..cterized by 

 marked toxin formation was attempted, even before Behring's dis- 

 covery, by Richet and Hericourt,' experimenting with cocci, and by 

 Babes,^ in the case of rabies; and the underlying thought had been the 

 basis of Toussaint's work upon anthrax. Microorganisms, however, 

 which exert their harmful action rather by the contents of the bacterial 

 cells than by secreted, soluble toxins, do not, so far as is known, pro- 

 duce antitoxins in the sera of immunized animals. The substances 

 which they call forth in the process are directed against the invading 

 organisms themselves in that they possess the power of destroying or 

 of causing dissolution of the specific germs used in their production. 



Such antibacterial sera are extensively used in the laboratory in the 

 passive immunization of animals against a large number of germs, and 

 are fairly effectual when used before, at the same time with, or soon after, 

 infection. Their therapeutic use in human disease, however, has, up 

 to the present time, been disappointing and their prophylactic and cura- 

 tive action has been almost invariably ineffect\ial or feeble at best, ex- 

 cept when the antibacterial sera could be brought in direct contact 

 with the germs, in closed cavities or locaUzed lesions. Thus, in epidemic 

 meningitis, such sera have proved extremely useful in the hands of 

 Flexner, when iajected directly into the spinal canal. 



ANTIBODIES AND THE SUBSTANCES GIVING RISE TO THEM 



In the foregoing sections we have seen that the process of active 

 immunization so changes the animal body that it becomes highly 

 resistant against an infection to which it had formerly in many in- 

 stances been dehcately susceptible. In the absence of visible anatomical 

 or histological changes accompanying the acquisition of this new power, 

 investigators, in order to account for it, were led to examine the physio- 

 logical properties of the body cells and fluids of immunized subjects. 

 While it was reasonable to suppose that all the cells and tissues were 

 affected by, or might have taken part in, a physiological change so 

 profoundly influencing the individual, the blood, because of its unques- 

 tionably close relation to inflammatory reactions, and because of the 



' Richet et Hericourt, Compt. rend, de I'acad. des sci., 1888. 

 2 Bohes et Lepp, Ann. de I'inst. Pasteur, 1889. 



