DIPHTHERIA. 221 







When the horses have been rendered sufficiently immune, 

 five or six liters of blood are withdrawn from the jugular 

 vein by means of a trocar, eight or ten days after the last 

 injection. The blood, after standing in the refrigerator, 

 yields a clear serum, which exhibits its maximum activity 

 eight or ten days after the injection of toxin. If the blood 

 is removed earlier, the serum is considerably weaker. 

 After the period of maximum activity, which persists for 

 several days, a gradual reduction takes place, which may 

 lead to complete disappearance of the antitoxin, unless 

 meanwhile new injections of toxin are made. For purposes 

 of preservation carbolic acid, in the proportion of 0.5 per 

 cent, is added to the antitoxin (Behring, Hochst serum), or 

 0.4 per cent, trikresol (Aronsohn, Schering serum), or a bit 

 of camphor (Pasteur Institute). 



It has been pointed out in the general section that 

 toxin and antitoxin, admixed in a test-tube, mutually 

 neutralize the activity of each other, so that when injected 

 simultaneously into guinea-pigs, no manifestations of disease 

 follow. In this mutual interaction a regular gradation is ob- 

 servable. In estimating the immunizing value of the cura- 

 tive serum Behring and.Ehrlich did not proceed from the 

 simple lethal dose, but from ten times that amount. They 

 designate as normal serum that of which o. I cu. cm. 

 suffices to neutralize ten times the lethal toxic dose, so that 

 a guinea-pig weighing 250 grams withstands the injection 

 without injury. One cubic centimeter of this serum repre- 

 sents a normal antitoxin-unit. The further calculation can be 

 readily made upon this basis. A serum, for instance, of 

 which o.oi cu. cm. neutralizes 10 times the lethal dose 

 represents 10 times the strength of normal serum that is, 

 I cu. cm. contains 10 normal antitoxin-units. Should 

 o.ooi cu. cm. of a serum suffice for this purpose, it repre- 

 sents a strength 100 times that of normal serum, and I 

 cu. cm. contains 100 normal antitoxin-units. 



Behring subsequently introduced a new method of cal- 

 culation. He designates as diphtheria normal antitoxin, 

 DAN 1 , that serum of which I cu. cm. neutralizes I cu. cm. 

 of diphtheria normal toxin, DTN 1 , = : -f- 25,000 M. These 

 figures he converts directly into weight of guinea-pigs, in 

 grams, M, by simply prefixing the minus sign. DAN 1 

 thus equals 25,000 M : that is, I cu. cm. of diphtheria 

 normal antitoxin is capable of neutralizing the lethal dose 



