PRECIPITINS 199 



The tissue in which the precipitin is formed is evidently not the 

 same in all cases. It depends on the nature of the precipitinogen and 

 the avenue of its introduction. When a foreign protein is introduced 

 parenterally, it has a predilection for certain tissues and this is dif- 

 ferent for different proteins. We only know at present that precipitins 

 are formed in organs rich in leukocytes. The mononuclear cell is 

 believed to be active in this function. It is probable that the endo- 

 thelial cells of the blood vessels also have this function. 



The precipitin can be thrown down in serum by one-third satura- 

 tion with ammonium sulphate and seems to be contained in the 

 euglobulin fraction. Attempts to obtain a protein-free precipitin have 

 failed; indeed, I think it highly probable that the precipitin is a pro- 

 tein. When heated to 70 degrees the serum no longer reacts with the 

 precipitinogen. 



The Precipitate. This results from a reaction between the pre- 

 cipitinogen and the precipitin. In case of bacterial nitrates, the precipi- 

 tate does not form directly on adding the precipitin to the precipi- 

 tinogen and it is customary to allow the mixture to stand at incubator 

 temperature for from two to twenty-four hours. The rapidity and 

 abundance of the formation of the precipitate depend on the richness 

 of the fluids in their respective active constituents. With animal pre- 

 cipitinogens the precipitate may appear immediately on the mixture of 

 the fluids. Precipitation takes place in neutral, feebly alkaline and 

 feebly acid solutions. When the acidity is due to organic acid or acid 

 salt, the action is promoted. In the complete absence of inorganic 

 salts the reaction fails. The precipitate consists essentially of pro- 

 tein. It contains no carbohydrate group, is insoluble in mineral acid 

 and alkali, and is highly resistant to the digestive ferments. There 

 seems no doubt that the bulk of the precipitate is derived from the 

 precipitin. Pick employed a precipitinogen which did not give the 

 biuret reaction and with this obtained a voluminous albuminous pre- 

 cipitate. Welch and Chapman obtained with a precipitinogen which 

 contained only 1 mg. of protein, a precipitate which contained 25.1 

 mg. of protein. Evidently, in these cases, the bulk of the precipitate 

 must come from the serum. However, this is not always true since 

 the bulk of the precipitate produced with lactosera consists of 

 casein. It is probable that in many instances substances other than the 



