REACTING POTENCY 263 



Combinations of human serum with horse anti-human serum: 



A zone precipitation reaction was only oijscrved in mixtines of nnclilnted 

 human serum with undikited horse anti-human serum. It is of considerable 

 importance, however, that zone reactions in formation of reacting factors did 

 not coincide with the precipitation curves. Thus, no reactions were obtained 

 in rabbits Avith mixtines of undiluted horse anti-human serum and undiluted 

 human serum and human serum diluted 1:2, 1:4, and 1:64. Positive results 

 were consistently observed with mixtures of undiluted horse anti-human 

 serum with human serum diluted 1:8, 1:16 and 1:32. 



Mixtures of horse serum with rabbit anti-horse serum: 



The precipitation zone reaction was only observed when rabbit anti-horse 

 serum was mixed with undiluted horse serum and horse serum diluted 1:2. 



No reactions were obtained in rabbits with mixtures of undiluted rabbit 

 anti-horse serum and with horse serum undiluted and diluted 1:5. Higher 

 dilutions of horse serum (1:20, 1:40 and 1:80) with undiluted rabbit anti- 

 horse serum gave strong reactions. 



Simultaneously with the foregoing studies on the zone reactions, 

 a series of experiments ^vas carried out in order to determine 

 Avhether the reacting potency of the antigen-j-antibody complexes 

 (oiild he neutralized by imminie sera similarly to neutralization 

 of bacterial actixe principles. In these attempts the complexes 

 were used for imnuuiization oi rabbits. The residting serum xvas 

 added to the ingredients separately or to the mixtines at various 

 times. It xvas imj)ossible, however, to obtain inactixation of the 

 reacting potency of the complexes. 



ORIGIN OF REACTING POTENCY OF SERUM PRECIPITATES 



Experiments xvere performed in order to determine xvhich of 

 the ingredients of serum+antiserum mixtures was responsible for 

 the reacting potency of the precipitates. Mixtures of some batches 

 of horse serum with anti-horse rabbit serum yielded potent pre- 

 cipitates xvhilst other batches of horse serum mixed xvith the same 

 anti-horse rabbit serum gave inactive precipitates. It appeared, 

 therefore, that the reacting potency xvas imparted by the precipi- 

 tinogen-containing serum. In another series of observations, hoxv- 

 ever, several batches of normal human serum mixed xvith the same 

 anti-human rabbit serum gave inactixe precipitates, but anti- 

 human anti-meninoococcus horse serum mixed xvith the same 

 batches of normal human serum yielded potent precipitates. Here 

 then, the reacting potency was, apparently, imparted by the 

 precipitin-containing antiserimi. It can be concluded, therefore, 



