ORIGIN OF PRECIPITATE. 



117 



While animal specificity is not destroyed when the albumins 

 Precipitin. are modified in the above manner or changed by tryptic diges- 

 tion or oxidation, Obermeyer and Pick have demonstrated that 

 their specificity is lost when an iodin, nitro or diazo group is inserted 

 into the proteid molecule. Immunization with such transformed proteid 

 compounds, e.g., xanthoprotein, can produce a precipitating serum which 

 will react with every xanthoprotein even in homologous animals. These 

 authors conclude that species specificity is probably dependent upon a 

 certain aromatic group of the proteid molecule. 



It is interesting to note that the proteid contained in the lens of the eye 

 belongs to this class of modified proteids which possess constitutional, 

 but no species specificity. A serum produced by immunization with lens 

 substance, will react with the proteid derived from the lens of any animal 

 but with no other animal proteid. 



In conclusion, the origin of the precipitate formed during the 

 Origin of precipitation reaction is of interest. When a very strong 

 Precipitate, precipitating serum is employed, the precipitinogen is so 

 greatly diluted that it no longer gives any of the chemical 

 reactions for proteids, but nevertheless yields a heavy precipitate when the 

 precipitating serum is added. This surely cannot come from the small 

 trace of proteid in the precipitinogen. Furthermore, if the immune serum 

 is diluted, the formed precipitate becomes comparatively weaker and dis- 

 appears entirely if dilution is increased. It is, therefore, generally considered 

 that the precipitate originates from the immune serum. 



