102 VACCINE AND SERUM THERAPY. 



By means of Gibson's or Banzhaf's method the serum is now 

 concentrated and refined. After this it is standardized, i. e., its 

 strength is determined. The United States government deter- 

 mines the unit of diphtheria anti-toxin in the United States, and 

 the Government requires that all diphtheria anti -toxin made by 

 manufacturers, having a United States government license, must 

 conform to this standard. This emphasizes the importance of 

 using only diphtheria anti-toxin made by manufacturers having a 

 United States Government license. Because the toxic value of 

 diphtheria toxin changes with ageing and other conditions, the 

 Hygienic Laboratory of the U. S. Public Health and Marine Hos- 

 pital Service, issues from time to time standard anti-toxic serum. 

 With this serum the strength of the toxin used in determining 

 the anti-toxic value of serum is gauged. The "immunity unit," 

 or unit of anti-toxin, is the amount of diphtheria anti-toxic serum 

 which will just neutralize one hundred times the smallest amount 

 of toxin necessary to kill a 250 gram guinea pig in four days. 

 The anti-toxic value of serum is determined by mixing one hun- 

 dred times the smallest fatal dose of fresh diphtheria toxin with 

 varying amounts of the diphtheria anti-toxin, allowing the mix- 

 tures to stand for fifteen minutes and then injecting them into 

 250 gram guinea pigs. The smallest amount of serum which will 

 protect a 250 gram guinea pig for more than four days against one 

 hundred times the smallest amount of diphtheria toxin necessary 

 to kill a 250 gram guinea pig, contains one unit of diphtheria anti- 

 toxin. 



At the time of bleeding immunized horses, that is after six 

 to eight weeks of immunization, the serum may contain from 

 one hundred to one thousand units of diphtheria anti-toxin per c. c. 

 The concentrated and refined diphtheria anti-toxin contains usually 

 from three hundred to two thousand units of anti-toxin per c. c. 

 The various manufacturers of diphtheria anti-toxin now furnish 

 their product in syringes containing a determined number of 

 units of anti-toxin. These packages are stamped so as to indicate 

 the date after which they can be exchanged, free of charge, for 

 more recently tested serum. Because of the degeneration in 

 potency of diphtheria anti-toxin in the fluid condition, it is desir- 

 able that no sera be used after the date when they are to be ex- 

 changed for new sera. 



