REACTING POTENCY 273 



mation of reacting factors /// vivo. The active sensitization^ to the 

 animal protein necessary for this phenomenon appeared in about 

 85 per cent of rabbits seven to eight days after a single intra- 



FiG. 30. Rabl)it sciisiti/cd 1)) single injection of horse serum six days previously, 

 received intradermal injection of 0.25 c.c. of undiluted B. typhosus active principles. 

 Twenty-four hoias later rabbit ^v•as injected intravenously \vith horse serinii. Appear- 

 ance of reaction at prepared skin site twenty-four hours after intravenous injection 

 of horse serimi. 



\'enous injection and it persisted as long as t^vo Aveeks Avithoiit 

 reinjection. Shorter incid)ation ]3eriods ^vere inadequate. A single 

 injection of 1 c.c. of horse serinn diluted 1:10 induced sensitiza- 



^ The term "sensitization" employed is limited to the type described. The term 

 "incubation period" refers to the interval of time between the last sensitizing and 

 first test injection. When repeated sensitizing injections were made, the true incuba 

 tion period \vhich \vas necessary remained imkno\\n. 



