154 VETERINARY BACTERIOLOGY 



Precipitins. The precipitins are quite analogous to the ag- 

 glutinins. They are formed as a result of the injection of a great 

 variety of proteins or protein derivatives. An antigen which in- 

 duces the development of precipitins is known as a predpitogen. 

 The precipitogen is doubtless the protein molecule itself. It con- 

 sists essentially of a binding or haptophore group only. The 

 precipitin seems to be formed in a manner similar to that already 

 described for agglutinins. It may be shown to consist of a 

 zymophore or precipitating group, and a haptophore or binding 

 group. The zymophore is easily destroyed by heat, but the hap- 

 tophore is thermostabile. A precipitin that has lost its zymophore 

 is known as a predpitoid. The precipitins are quite specific, but 

 group precipitation will take place when related proteins are 

 treated with a serum homologous to one of them. The blood- 

 sera of various ruminants, for example, exhibit group precipita- 

 tion. An antiserum homologous to human serum will precipitate 

 the serum of anthropoid apes. 



The work of Nuttall has showed quite definitely the limits 

 of group agglutination. He tested 900 kinds of blood, using in 

 all 30 antisera, and made a total of about 16,000 combinations. 

 He showed that, on the whole, the closer the relationship, the greater 

 the amount of common or group agglutination. For example, 

 he determined that the blood of apes of the old world would yield 

 a heavier precipitate with human antiserum than would that of 

 apes of the new world. Another exception to specificity has 

 been found in the protein of the crystalline lens: an antiserum 

 for the lens protein of man or the ox will produce a precipitate in 

 solution containing the lens protein from other animals not at all 

 closely related. It has been suggested that the cataract of the 

 eye may be due to the formation of an autoprecipitin for the 

 protein of the lens and its consequent partial coagulation or pre- 

 cipitation in situ. 



The mechanism of precipitation is not well understood, but 

 it is probably to be explained on the basis of certain facts of 

 colloid chemistry. The test is so delicate that a positive reaction 

 has been secured with a dilution of 1 : 100,000 of egg-white, while 

 the ordinary protein tests of the chemist fail to show 1 : 1000. 



Uses Made of the Precipitation Phenomenon. Several practical 



