ANTITOXINS 13 



in 1890 Behring showed that the serum of rabbits 

 artificially immunized against diphtheria was able 

 to confer a specific immunity against diphtheria 

 infections in other animals. He also demonstrated 

 that such a serum could be used therapeutically to 

 cure an infection already in progress. Such a serum, 

 although not bactericidal, retained its therapeutic 

 power for a considerable time. He believed that 

 the action of the serum was effected by a neu- 

 tralization of the bacterial toxin by an " antitoxic 

 serum constituent.'* The action was strictly spe- 

 cific, an antitoxic serum obtained after a diphtheria 

 infection protected only against diphtheria; one 

 derived from a tetanus animal, only against tetanus. 

 Subsequently Behring and Knorr showed that the 

 bacteria-free filtrate of the broth in which diph- 

 theria or tetanus bacilli had been grown was able 

 to kill certain animals; these animals exhibited all 

 the symptoms usually accompanying diphtheria or 

 tetanus poisoning. Minute amounts of these fil- 

 trates administered under certain conditions con- 

 ferred a specific immunity against infection with 

 these organisms and also against poisoning by their 

 toxic products. After considerable experimental 

 work Behring and his collaborators devised an effect- 

 ive method of immunizing sheep and certain other 

 animals against diphtheria and against tetanus and 

 so produced antitoxic sera in considerable amounts. 

 Behring's publication was followed in the next 



