414 PATHOGENIC BACTERIA. 



of this stronger serum was described as an antitoxic unit, 

 and, of course, contained ten immunizing units. Still 

 later it was shown that the limits of strength were by- 

 no means reached, and he succeeded in making serums 

 three hundred times the normal strength, each cubic 

 centimeter of which contained 300 immunizing units, 

 or 30 antitoxic units. 



In the course of the development of strength in the 

 serum the exact meaning of " immunizing unit " grad- 

 ually became obscured, until it is at present an expres- 

 sion of strength rather than one of quantity. 



While it is difficult to define an immunizing unit, it is 

 not at all difficult for one skilled in laboratory technique 

 to determine the number present in a sample of serum. 

 There are three rules of practice: 



1. Determine accurately the least certainly fatal dose of a 

 sterile diphtheria toxin for a standard guinea-pig. 



2. Determine accurately the least quantity of the serum that 

 will protect a guinea-pig against ten times the determined least 

 certainly fatal dose of toxin. 



3. Express the required dose of antitoxic serum as a fraction of 

 a cubic centimeter and multiply it by ten. The result is one unit. 



Example : It is found that 0.01 c.cm. of toxin kills at least 9 

 out of 10 guinea-pigs. It is then regarded as the least certainly 

 fatal dose. Guinea-pigs receive ten times this dose (0.1 c.cm.) 

 and varying quantities of the serum, measured by dilution, say, 

 t^ c.cm.,^0 c.cm., ^ c.cm. The first two live. The frac- 

 tion ^(j is now multiplied by 10 ; 2 ^V(j X 10 = ^ = 1 unit, and 

 we find that each cubic centimeter of the serum contains 250 

 Units. 



The most accurate definition of an immunizing unit is: 

 ten times the least amount of antitoxic serum that will 

 protect a standard (300-gram) guinea-pig against ten 

 times the least certainly fatal dose of diphtheria toxin. 



The strongest serum I have ever obtained contained 

 1700 units per cubic centimeter. 



The correctness of the mode of testing just described 

 depends upon the ability of one unit of antitoxin exactly 

 to neutralize one unit, of toxin. Ehrlich ! points out, 



1 Klinisches JaJu'buch, 1897. 



