246 RECEPTORS OF THE SECOND ORDER 



precipitins is similar to that of agglutinins, and need not be 

 repeated. Substitute the corresponding 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 ac- 

 curate. 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 kind of blood present in a given spot 

 shown by chemical and spectroscopic tests to be blood, and 

 has been adopted as a legal test in countries where such rules 

 of procedure are applied. Similarly the test has been used 

 to identify the different kinds of meat in a sausage, and 

 different kinds of milk in a mixture. An extract of the 

 sausage is made and tested against the serum of an animal 

 previously treated with extract of horse meat, or hog meat, 

 or beef, etc., the specific precipitate occurring with the 

 specific serum. Such reactions have been obtained where 

 the protein to be tested was diluted 100,000 times and more. 

 Biological relationships and differences have been detected 

 by the reaction. Human immune serum shows no reaction 

 with the blood of any animals except to a slight extent with 

 that of various monkeys, most with the higher, very slight 

 with the lower Old World and scarcely any with New World 

 monkeys. 



It is a fact of theoretical interest mainly that if agglutinins 

 and precipitins themselves be injected into an animal they 

 will act as anfigeji^ and cause the formation of antiagglu- 

 tinins or antiprecipitins, which are therefore receptors of 

 the first order since they simply combine with these iniimune 

 bodies to neutralize their action, have only a combining or 



