SEROLOGY MANUFACTURE AND USE OF SERA AND VACCINES 265 



probability, not enough antitoxin to neutralize the one hundred fatal doses 

 of the toxin, and the eighth tube has, in all probability, a great excess of 

 antitoxin. The contents of one tube is injected into a guinea-pig, thus 

 requiring eight pigs. The animals are marked and kept under close 

 observation. The first, second and perhaps third die, showing that not 

 enough serum was added to neutralize the toxin. The fourth pig just 

 recovers, showing that the amount of serum added to the fourth tube 

 was sufficient to neutralize one hundred fatal doses of the toxin. This 



FIG. 65. Guinea-pigs in wire cages. These lively little animals are used in test- 

 ing the virulense of the diphtheria toxin which is injected into the horse and also for 

 the purpose of standardizing the antitoxin. The reasons why these animals are pre- 

 ferred are wholly biological and physiological. They propagate rapidly, are easily 

 kept, easily handled, and respond (biologically) to the tests applied. 



amount of serum (antitoxic) represents one unit. From this amount 

 or unit the quantities to be put into the containers are determined. 500, 

 1000, 2500 and 5000 unit quantities are put up, for the convenience of 

 physicians. 500 to 1000 units constitute an immunizing dose, given to 

 those who do not have diphtheria, but who have been exposed to the 

 disease. The larger doses are curative. The rule is to give large doses, 

 repeated as often as may be necessary. 1000 units are usually em- 

 ployed as immunizing doses; 3000, 5000 and 10,000 unit packages for 

 curative doses. 



