IMMUNITY. 347 



and that Horace, in his words " sero medicina 

 paratur " must have been expressing his con- 

 viction that the universal cure would be pre- 

 pared from serum. The principal service in 

 the development of serum therapy was rend- 

 ered, however, by Behring, who also knew how 

 to interest in the problem careful fellow-work- 

 ers like Kitasato, Ehrlich, Wernicke and others, 

 with the result that in the minds of many 

 workers the different varieties of curative se- 

 rum now seem to present the sole opportunity 

 for medical investigation, as some years ago, 

 under the influence of the tuberculin investi- 

 gation, was the case with the proteids. 



The starting point of the serum therapy is 

 the observation that animals, after being suc- 

 cessfully infected regain their health when 

 treated with a subcutaneous injection of the 

 serum of immunized animals. The result 

 varies according to the time which has elapsed 

 since the infection. If, for example, the muc- 

 ous membrane of the trachea is infected with 

 diphtheria bacilli, the injection of serum un- 

 der the skin causes, within 24-26 hours, the 

 fibrinous exudate of the mucous membrane to 

 be thrown off, stops the poisonous action, and 

 prevents further general infection. When the 

 infection is subcutaneous the fibrinous exu- 

 date in the derma is changed by the use of 



