THE DIPHTHERIA BACILLUS 397 



it will not prevent the development of postdiphtheritic paralyses. It 

 has both prophylactic and curative properties. It is not bacteriolytic 

 and exhibits no agglutinins for diphtheria bacilli. Nothing is definitely 

 known of the nature of diphtheria antitoxin. If the diphtheria toxin 

 is a ferment, the antitoxin would appear to be an antiferment. The 

 fact that it is precipitated with the globulin fraction of the blood serum 

 would suggest that it may be either closely related to the proteins or 

 a true protein itself. 



Diphtheria toxin varies considerably in its potency due to the fact 

 that it deteriorates; the antitoxin, on the contrary, is more stable. 

 Consequently for purposes of comparison and standardization a 

 standard antitoxin is used. Two such standard antitoxins are 

 recognized officially: one prepared by Ehrlich in Germany; the other 

 prepared by the United States Public Health Laboratory in Washing- 

 ton, D. C. Both of these antitoxins were prepared on a very large 

 scale and preserved in a cold, dark, dry place in packages of conveni- 

 ent size. When the supply of one or the other of the standards is 

 nearly exhausted a new lot of antitoxin will be prepared and carefully 

 compared with the old. Small amounts of the standard antitoxin 

 containing a definite number of antitoxin units are sent out regularly 

 by the central laboratories to interested laboratories for testing 

 purposes. 



Standardization of Antitoxin. The antitoxin unit may be defined 

 as "that amount of antitoxin which just suffices to protect a guinea- 

 pig of 250 grams weight against 100 times the minimal fatal dose of 

 diphtheria toxin." The process of standardization of antitoxin of 

 unknown potency is carried out in the following manner: diphtheria 

 toxin, prepared as described above, is mixed in gradually diminishing 

 amounts with a definite amount of the standard antitoxin (containing 

 a known number of antitoxic units) and allowed to stand for twenty 

 to thirty minutes to permit union of the toxin-antitoxin to take place. 

 The mixtures are then injected subcutaneously into guinea-pigs of 

 250 grams weight. The greatest dilution of toxin which kills a guinea- 

 pig in four days is said to be the L+ dose that amount of toxin 

 which will neutralize (say) 100 antitoxin units and leave an excess of 

 toxin just sufficient to kill the animal. Having found the L-f- dose 

 of toxin (which standardizes its toxicity in terms of standard anti- 

 toxin), the same process is repeated, using this L-f dose of toxin mixed 

 with gradually diminishing amounts of the antitoxin to be stan- 

 dardized. That dilution of antitoxin of unknown potency which will 



