2Q4 SPECIFIC MICRO-ORGANISMS 



property, is dissolved. This is precipitated by the addition of 

 dilute acetic acid, filtered out and again taken up in salt solu- 

 tion. It is carefully neutralized with sodium carbonate and 

 dialyzed for several hours against water to remove the inorganic 

 salts. The residue in the dialyzer is then passed through a 

 Berkfeld filter to sterilize it, a preservative is added, and it is 

 ready to be tested and put up in containers for distribution. 

 The final product contains 75 to 90 per cent of the original anti- 

 toxic strength and is only about one-third as bulky. The serum 

 albumin, euglobulin and nucleoprotein have also been to a large 

 extent eliminated in the process of concentration. 



The antitoxic strength of anti-diphtheritic serum is expressed 

 in immunity units and is ascertained by animal experimentation. 

 The von Behring unit is contained in ten times the amount of 

 serum required to protect a 250 gram guinea-pig perfectly from 

 the effects of ten times the dose of fresh diphtheria toxin which 

 kills a similar guinea-pig in four days. The dose of toxin is 

 first ascertained by trial on guinea-pigs and the dose necessary 

 to kill in four days (minimum lethal dose) determined. Ten 

 times this quantity is then injected along with varying doses of 

 antitoxic serum into a series of guinea-pigs until the quantity 

 of serum, which not only saves the animal but prevents loss of 

 weight and local induration at the site of injection, has been 

 ascertained. Ten times this amount contains one immunity unit. 



Ehrlich has carefully standardized an antitoxic serum and 

 has preserved it as a dry powder, of which one gram contains 

 1700 immunity units. This standard is now employed as the 

 official standard for comparison in the United States. In stand- 

 ardizing an antitoxin by the Ehrlich method, one unit of the 

 standard antitoxin is injected along with various quantities of a 

 toxin to ascertain how much of the latter is required so that the 

 animal dies after four days. This dose of toxin, which when 

 combined with one unit of the standard antitoxin, kills a 250 

 gram guinea-pig in four days is called the L + dose. One next 

 injects this L + dose along with varying quantities of the new 



