574 MICRO-ORGANISMS AND DISEASE [chap. 



effect of an already-established infection in a normal (unpre- 

 pared) animal. 



This immunising and curative power of the blood-serum 

 of the actively immunised animal body is commensurate with 

 the degree of immunisation, so that the blood-serum of an 

 animal in which the active immunisation has been carried 

 to a high degree has itself measurably greater immuni- 

 sing and curative power than the blood-serum of an 

 animal not immunised to the same degree (Diphtheria, 

 Tetanus). 



Behring uses as the standard for measuring (in diphtheria) 

 this potency of the blood-serum by taking as unit the 

 amount of serum required to completely neutralise a dose of 

 pure toxin that would produce death in ten guinea-pigs 

 each of about 300 grams weight in thirty to thirty-six hours. 

 As was mentioned in the chapter on Diphtheria, Roux and 

 Yersin, who first separated the diphtheria toxin elaborated 

 by the diphtheria bacilli in broth cultures, showed that the 

 injection into the subcutaneous tissue of the guinea-pig of 

 a fatal dose of this pure toxin produces the same tumour at 

 the seat of injection and the same subsequent symptoms 

 and death of the animal as does the injection of the active 

 and living diphtheria bacilli. Behring's unit of potency of 

 diphtheria serum is the amount of serum required to inject — 

 antecedently or simultaneously, or shortly after — in order to 

 neutralise, i.e. prevent from producing tumour, disease, and 

 death, a tenfold fatal dose of toxin in a guinea-pig of 300 

 grams weight. This measure of the serum is then its 

 antitoxic potency. Behring has carried the active immuni- 

 sation against diphtheria of animals : sheep, goat, to 

 such a high degree that the (diphtheria) antitoxic 

 potency of the serum of these animals reaches the high 



