ANTISERA 37 



blood-serum, as described below, a large amount of 

 blood is drawn off, and prepared for therapeutic pur- 

 poses. The same horse can be used over and over 

 affain for the formation of antitoxin, if allowed a 

 sufficient period for recuperation" after each bleeding. 

 The formation of a sufficiently potent antitoxic serum 

 usually requires from six weeks to two months. 



The strength of antitoxin is measured in units. 

 The antitoxic unit is that quantity of horse's serum 

 which will render harmless the injection of 100 mini- 

 mal fatal doses of toxin. It is now ascertained what 

 is the smallest quantity of toxin which when mixed 

 and injected with 1 antitoxin unit will kill a guinea-pig 

 of 250 grammes in 4 or 5 days. This quantity of toxin 

 is then mixed with diif erent dilutions of the horse's 

 serimi and injected subcutaneously into several 

 guinea-pigs weighing 250 grammes each. Supposing 

 that 1/1000 c.c. of the antitoxin neutralizes the dose 

 of toxin so that the guinea-pig injected with this quan- 

 tity remains alive, then 1 c.c. of the horse's serum is 

 said to contain 1000 units of antitoxin. The strength 

 of the antitoxic serum having been ascertained in this 

 manner, it is placed in suitable quantities in syringes 

 for therapeutic use. All these procedures are of 

 course carried out under strict aseptic conditions. 

 Antidiphtheric serum, like other antisera, loses its 

 potency after variable lengths of time. 



