IMMUNITY. 109 



These protective substances are found also in other 

 fluids of the body than in the blood; they occur, in- 

 deed, in the substance of all cells to a greater or less 

 extent. How much of this is simply in solution from 

 the serum, and where the substances are formed, is not 

 definitely known. 



5. By the injection of the blood-serum of animals 

 which have previously passed through a specific disease 

 or have been inoculated with the bacterial products. 

 The first, probably, to think of the possibility of effect- 

 ing this was Raynaud, who, in 1877, showed that the 

 injection of large quantities of serum derived from a 

 vaccinated calf into an animal prevented its successful 

 vaccination. Hericourt, Richet, and others demon- 

 strated the same thing for other diseases. The results 

 obtained by Behring and Kitasato upon diphtheria and 

 tetanus, where, indeed, the serum prevented the action 

 of the poisons rather than the direct development of the 

 bacteria, gave a still greater impetus to these investiga- 

 tions. 



The immunity produced by these substances affects 

 the entire body, as is only natural, since the blood into 

 which they are absorbed is distributed everywhere. 

 When the immunity is but slight, infection may take 

 place in the more sensitive regions and still be im- 

 possible in those tissues having more natural resistance. 

 If the serum is injected into other animals or man the 

 immunity is greatest immediately after absorption, and 

 then declines, being rather quickly (in several weeks or 

 months), almost entirely lost, so that repeated injections 

 are required to maintain the immunity. This is dis- 

 tinctly in contrast to the immunity acquired after the 

 introduction of bacterial products, where the tissues 



