200 PRECIPITINS 



precipitinogen and the precipitin are carried down mechanically. When 

 a new dose of precipitinogen is injected into an animal whose blood 

 already contains the corresponding precipitin, the amount of the latter 

 is for a time markedly decreased. This leads to the inference that the 

 union between precipitinogen and precipitin takes place in the body, 

 but there is no evidence that a precipitate is formed in vivo. 



That both factors enter into the formation of the precipitate quan- 

 titatively is known, but it is not known whether or not the whole of 

 each factor exists in the product, nor is it known whether the reaction 

 is a physical or a chemical one. Evidently the presence of electro- 

 lytes is essential to the reaction. Quantitatively the reaction is spe- 

 cific. The serum of animals immunized to one strain of the colon 

 bacillus precipitates the filtrate from this strain in higher dilution than 

 it does the filtrate from other strains, and there may be strains of the 

 colon bacillus whose filtrates it will not precipitate at all. It is quite 

 certain that the precipitation of filtrates agrees with the agglutination 

 of the living bacilli. It is difficult not to believe that the specificity 

 of both these reactions, precipitation and agglutination, is due to the 

 chemical structure of the protein molecule and that both are due to 

 definite cleavage of this molecule in the process of parenteral digestion. 



