478 BACTERIOLOGY. 



it is desired to ascertain the dose whicli is to be taken for 



an animal weighing 300 g. on the basis of 0.05 c.c. per 250 



.^.,then: J 



300 : X :: 250 : 0.05 x = 0.06. 



Testing of Antitoxin. 



The strength of an antitoxic serum is expressed in immu- 

 nity units. An immunity unit may be defined as the amount 

 of antitoxin which is present in 10 times the amount of 

 serum that is just sufficient to protect a 250 g. guinea-pig 

 against 10 times the minimal fatal dose of toxin. In other 

 words, an immunity unit will theoretically protect against 

 100 times the minimum fatal dose. The protection extends 

 not merely to saving the life of the animal but must also 

 prevent local swelling, as well as variation in temperature 

 and in body-weight. If, for example, 0.1 c.c. of serum 

 protects a guinea pig, then 1 c.c. of that serum is said to 

 ■contain 1 immunity unit. Again, if 0.001 c.c. protects then 

 1 c.c. of such serum coiitains 100 immunity units. Serum 

 ■can be prepared of such strength that the astonishingly 

 small amount of 0.000,05 c.c. suffices to protect a guinea- 

 pig. This serum, therefore, contains 2000 immunity units 

 in 1 c.c. 



The usual method of testing antitoxin is to inject 

 subcutaneously into several guinea-pigs, each weighing 

 about 250 g. ; mixtures of 10 times the minimum fatal dose 

 ■of toxin and variable amounts of antitoxin. The several 

 mixtures are made up to the same volume by the addition 

 of sterile physiological salt solution (0.75 per cent. NaCl). 

 The amount of antitoxin (a) which is just sufficient to pre- 

 vent any ill effects, even local edema, represents A of an 

 immunity unit. Hence, ~ represents the number of im- 

 munity units present in 1 c.c. of the serUm. 



