118 BACTERIOLOGY FOR NURSES 



method of concentrating such a serum has been devised which 

 not only reduces its bulk but is said to lessen the probability of 

 serum sickness. 



After a short period of rest a horse that has been used for the 

 production of antitoxin may be used in the same manner for a 

 further supply. Given three months rest each year a horse is 

 said to furnish a high-grade serum for three or four years. 



Standardization of Antitoxin. At first antitoxin serum was 

 administered in so many cubic centimeter doses, but since some 

 horses produce a much more powerful serum than others the results 

 were altogether irregular. The adoption of a standard unit, 

 therefore, was deemed necessary in order to obtain a certain 

 degree of accuracy in dosage. As a result the standard unit of 

 antitoxin was fixed as the smallest amount that will just neutralize 

 one hundred times the amount of toxin that will kill a guinea pig 

 weighing 250 grams in four days. 



In order to standardize an antitoxin it is necessary to have a 

 standard toxin against which to test it. Since toxin quickly dete- 

 riorates a standard antitoxin is supplied in small quantities by the 

 Hygienic Laboratory at Washington to the various manufacturers 

 for this purpose. A series of guinea pigs each weighing about 250 

 grams are inoculated with 1 BT& of the standard antitoxin plus 

 varying amounts of toxin. In this way the L+ (limes death) 

 dose, which is the amount of toxin plus. 1 dc. of antitoxin required 

 to kill a guinea pig in four days, is obtained. Having determined 

 the dose of toxin that just neutralizes the standard antitoxin this 

 constant dose of toxin plus increasing amounts of the new antitoxin 

 is injected into another series of 250 gram guinea pigs. At the 

 end of four days it will be found that those receiving the smallest 

 amounts of antitoxin have died and that the larger amounts have 

 protected the animals. The strength of the antitoxin is estimated 

 from the smallest protective amount. Thus if a guinea pig receiving 

 .003 c.c. of serum had died and the next of the series receiving 

 .004 c.c. had died also, but the third which had received .005 c.c. 

 and likewise all the others receiving larger doses had been protected, 

 then the serum would be said to contain 200 units of antitoxin 



