218 INFECTION AND IMMUNITY 



Antitoxin is fairly stable and if kept in a cool, dark place, may re- 

 main active, with but slight deterioration, for as long as a year. Kept 

 in a dry state, in vacuo, over anhydrous phosphoric acid, by the method 

 of Ehrlich, it retains its strength indefinitely. 



Standardization. Antitoxin units being measured in terms of toxin, 

 uniformity of measurement necessitates the possession by the various 

 laboratories of a uniform toxin. Antitoxin being more stable than 

 toxin, uniformity of toxin is obtained by means of a standard antitoxin 

 distributed from a central laboratory. This was first done by Ehrlich 

 in Germany, and is now done for the United States by the Public Health 

 and Marine Hospital Service laboratories. Bottles of the distributed 

 antitoxin are marked with the number of units contained in each c.c. 

 Dilutions of this are mixed with varying quantities of the toxin to be 

 tested, the mixtures are allowed to stand for 15 minutes to permit union 

 of the two elements, and injections into guinea-pigs of 250 grams weight 

 are made. .Thus the L + dose of the toxin is determined. (The L + dose 

 [p. 208] is the quantity of poison not only sufficient to neutralize one 

 antitoxin unit, 1 but to contain an excess beyond this sufficient to kill a 

 guinea-pig of 250 grams in 4 to 5 days. L + is chosen rather than L , the 

 simple neutralizing dose, because of the difference between toxins in 

 their contents of toxoid and toxon. 2 ) 



The L + dose of the toxin having thus been determined, this quantity 

 is mixed with varying dilutions of the unknown antitoxin. 3 Thus, given 

 an antitoxin in which 300 to 400 units to the c.c. are suspected, dilutions 

 of 1 : 200, 1 : 250, 1 : 300, etc., are made. One c.c. of each of these is 

 mixed with the L + dose of the toxin, and the mixtures are injected into 

 guinea-pigs of about 250 grams. If the guinea-pig receiving L + plus 

 the 1 : 250 dilution lives and the one receiving L + plus the 1 : 300 dilu- 

 tion dies in the given time, we know that the unit sought must lie be- 

 tween these two values, and further similar experiments will easily 

 limit it more exactly. The possibility of error in carrying out such 



1 The older definition of a unit of diphtheria antitoxin is the quantity of anti- 

 toxin sufficient to protect a guinea-pig of 250 grams against 100 times the fatal dose 

 of diphtheria toxin. This, however, holds true only if we are dealing with normal 

 toxins and antitoxins as at first devised by Behring. In the conditions under which 

 the measurements are made at present, however, this definition must be revised as 

 follows: A unit of antitoxin is that amount of antitoxin which will save the life of a 

 guinea-pig if injected together with an L+ dose of the toxin. 



2 Madsen, in Kraus u. Levaditi, "Handbuch," etc., 1907. 



3 Donitz, "Die Werthbem. der Heilsera," in Kolle u. Wassermann. 



