IMMUNOLOGICAL BEHAVIOR Ql 



to hatdi. This tact constituted a serious handicaj) in attempts to 

 (letcnnine the ability ot a seiiiin to neutralize the skin-prepara- 

 tory factors. However, in carrying out the experiments on a large 

 series of rabbits it was clearly seen that the skin-preparatory fac- 

 tors could be specifically neutralized by homologous immiuie sera. 



In studies on serum neutralization of skin-preparatory factors, 

 in iG per cent of rabbits no reactions were obtained cither in sites 

 injected with bacterial filtrates alone or mixtures of bacterial fil- 

 trates with immune sera. A certain number of normal rabbits 

 receiving the injection of the toxic filtrates alone and especially 

 of B. I\Ij/iosus tryptic digest broth filtrates, may remain resistant 

 to the phenomenon. As ^vill be seen in a sidjsequent chapter, the 

 above figine is the approximate percentage of spontaneously re- 

 sistant animals and. therefore, can not be necessarily ascribed to 

 the iLse of the immune serimi. There is a suggestive possibility, 

 howe\'er, that Avhen a large amotnit of seriun is used in mixture 

 Avith the skin-preparatory factors, a part of it may diffirse into the 

 sites prepared with the toxic filtrate alone and thus, be respon- 

 sible for negative results in control sites in rabbits normally 

 susceptible to the phenomenon. This suggestion comes from the 

 casual observation that in some rabbits in which the site injected 

 with the toxic filtrate alone is too close to the site injected with 

 a mixttne of the toxic filtrate ^vith the immune serum, there 

 appears at the toxic filtrate site a reaction irregular in shape. 

 The irregularity may invohe either the entire border of the reac- 

 tion or only a part of it. 



The results in the remaining animals (those sho^ving reactions 

 in the control sites) can be summarized as follows: 



There were two types of response to the injection of mixtures 

 of specific immune sera ^\■ith the skin-preparatory factors. Out of 

 a total of 316 tests with mixtures of B. typhosus filtrate and 

 homologous sera, there were 135 neutralizations by the sera in 

 various dilutions, and 181 failures to neutralize. This demon- 

 strates the fact that neutralization of B. typhosus skin-preparatory 

 factors by specific sera can be induced at will in a large propor- 

 tion of experimental rabbits (about 42.5 per cent) . Varying the 

 site of the injection remains without influence on the results, 

 provided several multiples of the minimal skin-preparatory doses 

 are used. The filtrate was diluted 1:2 in these experiments. This 

 dilution combined Avith an intravenous injection of 2 c.c. of the 

 filtrate per kilo of body weight, represented at least 32 skin-pre- 



