266 RECEPTORS OF THE SECOND ORDER 



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 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 w^ith 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 mitigens and cause the formation of antiagglii- 

 tinins or antiprecipitins, which are therefore receptors of the 

 first order, since they simply combine with these immune 

 bodies to neutralize their action, have only a combining or 

 haptophore group. Also if agglutinins or precipitins be 

 heated to the proper temperature they may retain their 

 combining power but cause no agglutination or precipitation, 

 i. e., they are converted into agglutinoid or precipitinoid 

 respectively after the analogy of toxin anji toxoid. 



Precipitins like agglutinins possess at least two groups— 

 a combining or haptophore group and a precipitating (some- 

 times called zymophore) group. Hence they are somewhat 

 more complex than antitoxins or antienzymes which have 

 a combining group only. For this reason Ehrlich classes 

 agglutinins and precipitins as receptors or haptins of the 

 second order. 



