REACTIxXG POTENCY 275 



upon retest Avith human, guinea pig, rabbit, sheej) and goat sera. 

 Similarly, rabbits sensitized to luniian sertnn showed reactions 

 ^vith human serum but failed to respond to horse serum; and 





Fig. 32. Ral)bit sensitized by single injection ot horse seiiini six davs pieviouslv, 

 received intradermal injection of 0.25 c.c. of undiluted B. typliosus active principles. 

 T^\•enty-folu■ hours later, rabbit \vas injected intravenously ^vith horse serinn. Appear- 

 ance of reaction at prepared skin site forty-five minutes after intravenous injection 

 of horse serum showing severe perivascular inflammation and thrombosis. 



those sensitized to egg albumin did not react with horse serum. 

 However, the specificity displayed by serum proteins ot various 

 animal species did not hold ^vhen repeated sensitizations ^vith 

 large doses ^vere extended oxer prolonged periods of time. Thus, 

 rabbits sensitized by foiu' intraxenous injections of luidiluted 

 horse serum, in doses of 1 c.c. to 4 c.c. per kilo of body xveight, 

 reacted Avith sheep serinn after an incubation period of txventy- 

 two days, although failing to respond to guinea pig serum. Sera 

 of these rabbits ol3tained before the test injection xvere titrated 

 for precipitins against sheep and guinea pig sera. They gave pre- 

 cipitation with sheep serimi diluted 1 :20 and no precipitation Avith 



