466 ILLINOIS STATE ACADEMY OF SCIENCE 



Three years later, in 1905, Theobald Smith reported an 

 interesting observation. Guinea pigs have always been 

 used to test and standardize diphtheria toxin and anti- 

 toxin. Bearing this in mind, it can be understood readily 

 how vital it is that the susceptibility of the animals used 

 in these tests should vary as little as possible. Smith 

 noticed that guinea pigs bought in the open market were 

 less resistant to toxin than those he raised in his labora- 

 tory. He also noticed that the resistance of his own ani- 

 mals varied. He at first attributed these variations to 

 individual susceptibility or to differences in food and 

 environment. For the time being he ruled out any 

 preliminary treatment of the animals as a cause of 

 variations in their immunity. Continued irregTilarities 

 in his tests seemed to have disturbed Smith's mind, for 

 he studied more intensely and carefully the effect of 

 preliminary treatment of the female parent with refer- 

 ence to the antitoxin content of the blood of the offspring. 

 Two years later, in 1907, he reported that preliminary 

 treatment of female guinea pigs with TA mixtures not 

 only immunized them, but the offspring of these treated 

 mothers were jDassively immunized. In addition he 

 demonstrated conclusively that this passive immunity 

 of the offspring is a correct measure of the active im- 

 munity of the mother guinea pig. This discovery was ex- 

 ceedingly important, for by testing the successive litters 

 of guinea pigs he was enabled to watch the persistance 

 of the active immunity in the parent gniinea pig. Behring 

 had maintained that the immunity of the animal parents 

 had absolutely no influence on the susceptibility of their 

 offspring. This fiat of Behring undoubtedly adversely 

 influenced research workers the world over. 



Smith found that this active immunity lasted for sev- 

 eral years. He also showed in this paper of 1907 that 

 tliere is apparently no relation between the severity of the 

 reactions produced by the immunizing agents used and 

 amount of active immunity resulting. Animals that were 

 just able to survive a most powerful dose of toxin did not 

 thereby possess aiw immunity, while the injection of a 

 single dose of a mixture of toxin and antitoxin that 

 caused no local or general disturbance did confer a 



