DIPHTHERIA ANTITOXIN. 407 



bacilli either in a test-tube or at the site of inoculation 

 in the living animal body. 



This serum with which we have been experimenting 

 is the so-called "diphtheria antitoxin" or "antidiph- 

 theritic serum." 



For practical purposes, it is obtained from horses, 

 the animals being treated with gradually increasing 

 doses of diphtheria toxin until they are able to with- 

 stand enormous multiples of the ordinarily fatal dose. 

 When this point is reached, the protective body — the 

 antitoxin — is present in the blood in such large quan- 

 tities that the serum may be successfully emjiloyed in 

 the treatment of diphtheria in human beings — i. e., as 

 an antidote to the diphtheria toxin that is produced by 

 the growing bacteria in the throat, or elsewhere, and 

 distributed through the body by the circulating blood. 



For purposes of comparison, the protective strengths 

 of antitoxic serums are expressed in terms of units; 

 a unit being the amount of antitoxic serum which 

 will so neutralize 100 minimum fatal doses of toxin 

 for a guinea-pig of 260 grammes weight that the latter 

 will live at least four days after the mixture has been 

 injected subcutaneously. If, for example, a guinea- 

 pig of about 250 grammes weight be subcutaneously 

 injected with a mixture of 100 fatal doses of toxin and 

 0.01 C.C. of antitoxic serum, and live four days, it is 

 evident that the protective unit is 0.01 c.c. of the serum, 

 and that, expressed with regard to volume, each cubic 

 centimetre of such serum contains at least 100 antitoxic 

 units ; if 0.005 c.c. of the serum neutralized the 1 00 

 toxic doses, then the unit is represented by 0.005 c.c, 

 and 200 protective units will be contained in a cubic 

 centimetre of the serum, and so on. 



