260 RECEPTORS OF THE FIRST ORDER 



cannula. The serum from this blood with the addition of an 

 antiseptic (0.5 per cent phenol, tricresol, etc.) constitutes 

 " an tidiphth critic serum" or "antitetanic serum," etc. All 

 sera which are put on the market must conform to definite 

 standards of strength expressed in "units" as determined 

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

 diphtheria antitoxin in the United States is an amount 

 equivalent to 1 cc 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." 

 Specifically stated, a "unit" of antitoxin contains approxi- 

 mately the amount which would protect a 250 g. guinea- 

 pig from 100 minimum lethal doses of diphtheria toxin, or 

 protect 100 guinea-pigs weighing 250 g. each from 1 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 four 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 g. guinea-pig for ninety-six hours against 

 the standard test dose'' of the standard toxin. The standard 

 test dose is 100 M. L. D. of toxin for a 350 g. guinea-pig. 

 To express it another way, one could say that a "unit" of 

 tetanus antitoxin would protect 1000 350 g. 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 cc 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 

 antitoxins so that the exact amount of "units" per cubic 

 centimeter may be known. 



