CHAPTER XXV 



SPECIFIC PRECIPITINS 



History. In 1897 Kraus discovered that the blood serum of ani- 

 mals immunized to a given bacterium gives a precipitate when mixed 

 with a germ-free nitrate of a culture of that organism. Further study 

 showed that this reaction is specific. Cholera serum precipitates only 

 cholera filtrates ; typhoid serum only typhoid filtrates, etc. Two years 

 later Tchistowitsch and Bordet ascertained that this is a general pro- 

 tein reaction. A rabbit which has had several injections of horse 

 serum yields a serum which precipitates horse serum and no other. 

 One treated with cow's milk furnishes a serum which precipitates 

 cow's milk and no other. One treated with the white of the hen's egg 

 precipitates egg albumin and no other protein. This test is now used 

 practically in the identification of blood stains, in the detection of 

 mixed meats and in the study of the blood relationship of animals. 



If a blood stain be dissolved in salt solution and then mixed with 

 the blood serum of a rabbit which has been repeatedly treated with 

 human blood, a precipitate will form if the stain is human blood. If 

 a Hamburger steak be extracted with water, the clear filtrate will be 

 precipitated with the blood of an animal which has been treated with 

 horse's blood provided the steak contains horse meat. The meat of 

 any other animal in the steak can be detected in the same way. If 

 the steak contains beef and horse meat, its extract will precipitate the 

 serum of an animal treated with extract of horse meat also the serum 

 of one which has been treated with an extract of beef. The serum 

 of a rabbit treated with human blood will give a marked precipitate 

 even in a high dilution of human and a slighter one in undiluted chim- 

 panzee's blood serum. This supplies a method of detecting and iden- 

 tifying different proteins in quantities too small to be detected in any 

 other way. 



The Precipitinogen. The substance which is injected into the ani- 

 mal is known as the precipitinogen (producer of the precipitin). Pre- 

 cipitinogens are bacterial, vegetable, and animal. A culture of the 



