IMMUNITY 13 



Standardisation of Antitoxins. — Shortly after the 

 therapeutic application of antitoxin, it became ap- 

 parent that no two sera, though similarly produced, 

 could have exactly the same protective value. It was 

 necessary, therefore, to establish some standard by 

 which the approximate strength of a given antitoxin 

 could be estimated. Von Behring, and later Ehrlich, 

 attempted to do this by determining the quantity of 

 immune sera which was needed to protect a guinea- 

 pig of known weight against a definite dose of a 

 standard poison. They ascertained the quantity of 

 a standard toxin bullion which was able to kill a 

 guinea-pig of 250 grams in from four to five days, 

 and called this the "toxin unit." They spoke of a 

 toxin bullion which contained one hundred such 

 toxin units in a cubic centimetre as a "normal 

 toxin solution," and designated as "normal anti- 

 toxin" a serum capable of neutralising, cubic 

 centimetre for cubic centimetre, the normal poison. 

 A cubic centimetre of such an antitoxic serum was, 

 therefore, sufficient to neutralise one hundred toxic 

 units, and was spoken of as an " antitoxin unit " ; in 

 other words, a unit of antitoxic serum is that 

 quantity of serum which is able to protect a guinea- 

 pig of 250 grams weight against one hundred lethal 

 doses of toxin. 



Ehrlich's Side-Chain Theory.— The extensive re- 

 searches of Ehrlich into the nature of toxin-antitoxin 

 reaction led him to believe that the two bodies under- 

 went chemical union, forming a neutral compound. 



