DIPHTHERIA. 233 



is allowed to coagulate, and remains upon ice for two 

 days or so, that the clear serum may be pipetted off. 

 This serum is the antitoxic serum. It does not always 

 materialize according to the desires of the experimenter, 

 sometimes proving unexpectedly strong in a short time, 

 sometimes unexpectedly weak after months of patient 

 preparation. 



The strength of the serum is expressed in what are 

 known as immunising units. This denomination origin- 

 ated with Behring, whose original or normal serum was 

 of such strength that o. i c. cm. of it would protect against 

 the ten-times fatal dose of toxin when simultaneously in- 

 jected into guinea-pigs. Each cubic centimeter of this 

 normal serum he called an immunising unit. Later it 

 was shown that the strength of the serum could easily be 

 increased tenfold, so that o.oi c.cm. of the serum would 

 protect the guinea-pig against the ten-times fatal dose. 

 Bach cubic centimeter of this stronger serum was de- 

 scribed as an antitoxic unit, and of course contained 

 ten immunizing units. Still later it was shown that 

 the limits were by no means reached, and he succeeded 

 in making serums as much as three hundred times the 

 normal strength, each cubic centimeter of which con- 

 tained 300 immunizing units or 30 antitoxic units. 



The serums ordinarily sold are of three strengths 600 

 units in 10 c.cm., 1000 units in 10 c.cm., and 1500 units 

 in 10 c.cm. The weaker strength is used for immunizing 

 healthy children and adults who come in contact with 

 the contagium. The stronger serums are for treatment. 

 There is, of course, no way of estimating the amount of 

 toxin in the blood of a child suffering with diphtheria, 

 and therefore no accurate method of determining exactly 

 how much antitoxin should be given. Khrlich asserts 

 that less than 500 units is valueless: 10 c.cm. is probably 

 an average dose, and, as the remedy seems harmless, it is 

 better to err on the side of too much than on that of 

 too little. 



The largest collection of statistics upon the results of 



