140 LABORATORY COURSE IN SERUM STUDY 



uct, the "toxoid." The toxoid is not poisonous for guinea pigs, 

 and therefore the volume of original broth necessary to kill guinea 

 pigs of the required weight increases. Now if this deterioration 

 into toxoid implied at the same time diminution of neutralizing 

 power for antitoxin the titration could be easily adjusted by simply 

 remeasuring the minimal lethal dose for the particular solution 

 used and proceeding on this basis merely with a changed unit. 

 However, in old toxin solutions, although the minimal lethal 

 dose is larger, the neutralizing value of this poison for antitoxin 

 has changed very little or not at all. In other words, the toxin 

 derivatives or toxoids, although no longer poisonous to the 

 guinea pig, still retain their neutralizing power for the antitoxin. 

 It is obvious therefore that no constant standard could be obtained 

 merely by measuring the minimal lethal dose of any toxin broth 

 and measuring the antitoxin unit against 100 such minimal fatal 

 doses. The amount of toxoid would be alike in no two toxin filtrates 

 nor in the same filtrate at different times, and the antitoxin unit 

 which I determined against 100 MLD's of a toxin brothl have pro- 

 duced in my laboratory might be far different in antitoxic potency 

 from a unit similarly determined in another laboratory from an- 

 other toxin broth containing an entirely different proportion of 

 true toxin and non-poisonous antitoxin-neutralizing toxoids. 



In consequence a different system has had to be worked out 

 largely by the laborious investigations of Ehrlich. Ehrlich 

 found that the antitoxin was very much more stable than the 

 toxin. If dried, reduced to a powder, and preserved in vacuo 

 in a small glass U-tube over phosphoric anhydride in the cold 

 and in the dark, such dried serum would preserve its antitoxic 

 value for a long time and no deterioration comparable to that tak- 

 ing place in the toxin would occur. Originally Ehrlich prepared 

 tubes in this way, each one containing antitoxin powder of a 

 potency of 1700 antitoxin units to the gram, these antitoxin 

 units being measured against one of the toxins in his possession 

 representing the amount necessary to neutralize 100 MLD's of 

 this toxin. Against this powder then from time to time other 

 toxins are measured and the quantity determined which mixed 



