PRECIPITINS 265 



the experiments were tried of "immunizing" an animal not 

 against a bacterium or its filtered culture, but against 

 (colloidal) solutions of proteins, such as white of egg, casein 

 of milk, proteins of meat and of blood serum, vegetable 

 proteins, etc. It was ascertained that in all these cases the 

 animal's serum contains a substance which causes a pre- 

 cipitate with solutions of the protein used for immunization. 

 The number of such precipitating serums that have been 

 made experimentally is very large and it appears that pro- 

 tein from any source when properly introduced into the 

 blood or tissues of an animal will cause the formation of a 

 precipitating substance for its solutions. This substance is 

 known technically as a ''precipitin.'^ The protein used as 

 antigen to stimulate its formation, or some part of the pro- 

 tein molecule (haptophore group), which acts as stimulus 

 to the cell is spoken of as a "precipitinogen," both terms 

 after the analogy of "agglutinin" and "agglutinogen." In 

 fact the specific precipitation and agglutination are strictly 

 analogous phenomena. Precipitins act on proteins in 

 (colloidal) solution and cause them to settle out, agglutinins 

 act on substances within cells, which cells are in suspension 

 in a fluid and cause the cells to settle out. Ehrlich's theory 

 of the formation of precipitins is similar to that of aggluti- 

 nins and need not be repeated. Substitute the correspond- 

 ing words in the theory of formation of agglutinins as above 

 given and the theory applies. 



The precipitin reaction has not found much practical use 

 in bacteriology, largely because the "agglutination test" 

 takes its place as simpler of performance and just as 

 accurate. The reaction is, however, generally applicable to 

 filtrates of bacterial cultures and could be used if needed. 

 The so-called "mallease" reaction in glanders is an instance. 



Precipitins find their greatest usefulness in legal medi- 

 cine and in food adulteration work. As was noted above, 

 if animals, rabbits for example, are immunized with the 

 blood of another animal (human beings) precipitins are 

 developed which are specific for the injected blood with 

 proper dilution. This forms an extremely valuable means 

 of determining the hind of blood present in a given spot 



