36 TOXINES AND ANTITOXINES. 



The single lethal dose is defined by EHRLICH as that amount of 

 toxine, expressed in c.c. of the poison solution or in grammes 

 of the solid poison, that is just sufficient to kill a guinea-pig 

 weighing 250 grms. (an animal about six weeks old) within 

 four or five days. This dose is the physiological toxic unit. 



The definition of normal toxine, as fixed by v. BEHRING, is a 

 solution of poison that contains 100 lethal doses in 1 c.c. The 

 term "normal toxine" is abbreviated by v. BEHRING into 

 DTN, M 250 (Dijihtherietoxin normal einfach, Meerschweinchen 

 von 250 grms. i.e., single normal diphtheria toxine, guinea- 

 pigs of250 grms.). 



Now the measurement of the strength of antitoxine solutions 

 is based upon this arbitrary toxic unit. A " single " serum is 

 one, 1 c.c. of which is capable of neutralising 1 c.c. of the normal 

 toxine i.e., 100 toxic units. This amount (1 c.c. of the single 

 serum) is the antitoxine unit, the so-called immunity unit, briefly 

 designated I.E. (Immunitats-Einheit) 1 and has thus been em- 

 pirically established and retained (vide infra). 



When, then, a serum is first prepared against a new toxine, it 

 is invariably the rule in all the experiments made at the same 

 time to express the relationship of the toxine to the antitoxine 

 solution in terms of cubic centimetres. And since in the case 

 of this fresh toxine the ratio of toxic activity to the number of 

 c.c. used remains constant, there is also a constant relationship 

 between the toxic activity and the amount of antitoxine i.e., every 

 lethal dose invariably corresponds exactly with the same amount 

 of antitoxine solution, expressed in cubic centimetres. 



If, however, this toxine be allowed to stand for some time 

 and its relative value towards the serum be again determined, it 

 will be found that the conditions of quantitative combination 

 have materially changed in one respect. It is true that the 

 ratio of toxine to antitoxine solution, expressed in c.c., will 

 still remain constant i.e., that every c.c. of the toxine solution 

 requires the same amount of antitoxine solution as the fresh 

 toxine, but at the same time this quantity of toxine solution, 

 expressed in c.c., has considerably smaller toxic activity than an 

 equal amount of the fresh poison. If, on the other hand, a 

 determination be made of the amount of antitoxine required to 

 saturate one toxic unit, it will obviously be found that a con- 



1 Mad sen ("Constitution du poison diphth.," Ann. Past., xiii., 568, 

 1899) has introduced several other abbreviations. T = toxine unit; (T) 

 the amount of toxine bouillon in c.c. containing T ; I = immunity unit (in 

 German abbreviated to I.E.); and (I) the amount of serum in c.c. that 

 contains I. We shall occasionally use these abbreviations. 



