ANTITOXINS— ANTIENZYMES 241 



by the U. S. Hygienic Laboratory. In reality a "unit" of 

 diphtheria antitoxin in the United States is an amount 

 Equivalent to 1 c.c. of a given solution of a standard diph- 

 theria antitoxin which is kept at the above-mentioned 

 laboratory. This statement, of course, gives no definite idea 

 as to the amount of antitoxin actually in a "unit." Specifi- 

 cally stated, a "unit" of antitoxin contains approximately 

 the amount which would protect a 250-gram guinea-pig from 

 100 minimum lethal doses of diphtheria toxin, or protect 

 100 guinea-pigs weighing 250 grams each from one minimum 

 lethal dose each. The minimum lethal dose (M. L. D.) of 

 diphtheria toxin is the least amount that will kill a guinea- 

 pig of the size mentioned within foiu- days. Since toxins 

 on standing change into toxoids to a great extent, the amount 

 of antitoxin in a "unit," though protecting against 100 

 M. L. D., in reality would protect against about 200 M. L. D. 

 of toxin containing no toxoid. 



The official unit for tetanus antitoxin is somewhat dif- 

 ferent, since it is standardized against a standard toxin which 

 is likewise kept at the Hygienic Laboratory. The unit is 

 defined as "ten times the amount of antitoxin necessary to 

 protect a 350 gm. guinea-pig for 96 hours against the standard 

 test dose" of the standard toxin. The standard test dose is 

 100 M. L. D. of toxin for a 350 gm. guinea-pig. To express 

 it another way, one could say that a "unit" of tetanus anti- 

 toxin would protect one thousand 350 gm. guinea-pigs from 

 1 M. L. D. each of standard tetanus toxin. 



Various methods have been devised for increasing the 

 amount of antitoxin in 1 c.c. of solution by precipitating 

 out portions of the blood-serum proteins and at the same 

 time concentrating the antitoxin in smaller volume. It is 

 not considered necessary in a work of this character to enter 

 into these details nor to discuss the process of standardizing 

 antitoxin so that the exact content of "units" per c.c. may 

 be known. 



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