TOXINS AND ANTITOXINS 267 



serum, without the intervention of "phagocytes." 

 The fresh serum of other animals was next studied 

 and found to possess great bactericidal power, in 

 certain cases even in animals that were susceptible 

 to anthrax infection. These results pointed to the 

 defensive mechanism being mainly situated in the 

 fluids of the body and formed the basis of the 

 " humour al theory" as against the "phagocyte 

 theory." Nevertheless, it was clear that the com- 

 position of the serum is dependent primarily on 

 the activities of the different kinds of cells which 

 collectively constitute the body of an animal. 

 These experiments are referred to because they 

 formed the starting-point for researches which re- 

 sulted in the discovery of the curative properties 

 of serum, of "serum therapeutics." When it had 

 been found that bacteria could be destroyed by 

 serum the next step followed naturally, it was to 

 try if the serum of an animal which had recovered 

 from a disease possessed protective and curative 

 properties when injected into an otherwise suscep- 

 tible animal. 



It is an old observation that persons by re- 

 peated dosing may attain a considerable tolerance 

 to poisons and in certain cases may acquire a posi- 

 tive resistance to large doses of poisons which would 

 otherwise cause death. It was found by Behring 

 that the poison formed by the bacilli of diphtheria 

 (or tetanus) in laboratory cultures, if injected in 

 gradually increasing doses into susceptible animals, 



