R6LE OF PRECIPITINS IN IMMUNITY 319 



animals, from fishes to man, and a serum produced by immunization 

 with lens substance will react with the protein derived from the lens jof 

 any animal, but with no other animal proteid. This theory of species 

 specificity may, however, be carried a little further, for by carefully 

 freeing the organs from all blood and then using organic extracts for 

 inoculation, it is possible to produce serums that will yield a precipitate 

 best with the particular variety of organic extract (liver, kidney, etc.), 

 and a weak or no precipitate with extracts of other organs of the same 

 animal. 



Besides this animal specificity, precipitin reactions also demonstrate 

 the " constitutional specificity" of proteins. If, instead of using a pure 

 animal or plant albumin for immunization, variously altered albumins 

 are used (heated albumins, acid albumin, formaldehyd albumin, and the 

 like), the organism reacts by producing antibodies of a characteristic 

 nature, differing from those developed after inoculation with pure al- 

 bumin. For example, if a rabbit is immunized with normal horse serum, 

 the resulting immune serum will produce a precipitate when added to 

 pure horse serum, but not when added to horse serum that has been 

 heated. On the other hand, if a rabbit is inoculated with horse serum 

 that has been diluted and boiled for a short time, the resulting immune 

 serum will react not only with normal horse serum, but also with heated 

 serum ^and a group of its decomposition products with which the normal 

 immune serum ordinarily never produces a precipitate. 



This observation is of practical importance in detecting meat sub- 

 stitution by precipitin reactions. In order to render the detection dif- 

 ficult, the meat is commonly boiled ; with the aid of precipitins produced 

 by immunization with heated proteins, this fraud is more easily detected 

 than if a normal immune serum were used. 



Obermeyer and Pick have demonstrated that while animal specificity 

 is not destroyed when the albumins are modified by heat, tryptic di- 

 gestion, or oxidation, their specificity is lost when an iodin, nitro- or 

 diazo-group is inserted into the protein molecule. Immunization with 

 such transformed proteins, e. g., xanthoprotein, can produce a precipi- 

 tating serum that will react with every xanthoprotein, even that of 

 different animals. These investigators conclude that species specificity 

 is probably dependent upon a certain aromatic group of the protein 

 molecule. 



Role of Precipitins in Immunity. Precipitins are probably not truly 

 protective antibodies, like antitoxin and the lysins, but they are quite 

 similar to the agglutinins in being secondary products of cellular activ- 



