276 



INFECTION AND IMMUNITY 



diphtheria antitoxin, the L + dose of toxin is employed. The L + 

 dose is, however, in this case, denned as the smallest quantity of 

 tetanus toxin that will neutralize one-tenth of an immunity unit, 

 plus a quantity of toxin sufficient to kill a 350 gram guinea-pig 

 in just four days. At the Hygienic Laboratory at Washington, a 

 standard toxin and antitoxin are preserved under special conditions, 

 and standard toxin and antitoxin, arbitrary in their first establish- 

 ment, are kept constant by being measured against each other from 

 time "to time. In measuring the antitoxic serum thus preserved, at 

 the Hygienic Laboratory, a mixture of one-tenth of a unit of anti- 

 toxin and 100 minimal lethal doses of the standard toxin must 

 contain just enough free poison to kill the guinea-pig in four days. 

 This L + dose of the standard toxin is given out to those interested 

 commercially or otherwise in the production of antitoxin. 



In measuring an unknown antitoxic serum against this L + dose 

 of toxin, a large number of mixtures are made, each containing the 

 L + dose of the toxin and varying quantities of the antitoxin. Dilu- 

 tions must always be made with 0.85 per cent salt solution and the 

 total quantity injected into the animals should always be brought up 

 to 4 c.c. with salt solution in order to equalize the conditions of 

 concentration and pressure. The mixtures are then kept for one 

 hour at room temperature in diffused light. After this they are 

 subcutaneously injected into a series of guinea-pigs weighing from 

 300 to 400 grams. The following example of a test is taken from 

 the article by Rosenau and Anderson quoted above. 



In this series the guinea-pig, receiving 0.0015 c.c. of the antitoxin, 

 died in approximately four days; 0.0015 c.c. therefore represents 

 one-tenth of an immunity unit. 



In therapeutically employing antitoxin for prophylactic purposes, 

 above 1,500 units should be employed. 



