68 MODERN BIOLOGIC THERAPEUSIS 



guinea-pig of 250 grams weight in four days, 

 this quantity being termed the minimum lethal 

 dose (M.L.D.). The toxin most frequently 

 used for immunization has an M.L.D. of about 

 0.002 c.c. 



Immunizing the Horses After the relative 

 strength of the toxin has been determined it is 

 ready to use for immunizing the horses. Small 

 doses are used at first and are gradually in- 

 creased at suitable intervals until a large 

 amount of toxin can be tolerated, the object 

 being to establish a condition of hyperimmuni- 

 zation whereby a high concentration of anti- 

 bodies in the blood is attained. The injections 

 are made subcutaneously, using every precau- 

 tion to prevent bacterial infection. An injec- 

 tion of toxin is usually followed by a rise in 

 temperature in the animal. This soon passes 

 off and subsequent injections are made at 

 stated intervals. After several weeks' treat- 

 ment the horse is able to tolerate enormous 

 quantities of toxin sufficient to kill hundreds 

 of unimmunized horses. During the course of 

 immunization a sample of blood is taken from 

 time to time in order to determine its antoxic 

 value. 



