THE PRECIPITINS 233 



reactions " of the agglutinins ; that is to say, they are specific as 

 regards the antigens which bring them into existence, irrespective 

 of the source from which that antigen was derived. This appears 

 to be substantiated by experiments on purified and recrystallized 

 proteids, the precipitating sera for which show a high degree of 

 specificity. Too frequent recrystallization of proteids, however, 

 appears to injure their power of inducing the formation of 

 precipitins. 



Further, proteids which have been altered in character by 

 chemical processes (iodized, nitrified, or denitrified) will cause 

 the formation of precipitins which are specific for the transformed 

 molecules of proteid, no matter from what source they were 

 derived. These observations are of profound interest, since they 

 appear to show that it is possible by artificial means to alter com- 

 pletely the haptophore group of a proteid molecule. According 

 to Uhlenhuth, there is little or no specificity in the antisera pre- 

 pared against the proteids of the crystalline lens. An antiserum 

 prepared by injections of serum will precipitate all albuminous 

 fluids except a solution of the lens, which in its turn will not pre- 

 cipitate with serum. But an anticrystalline serum prepared by 

 the injection of ox lenses into rabbits will give precipitates with 

 lens solutions from mammals, birds, amphibia, and even fish. 



Hence, too, a practical point in connection with the employment 

 of precipitating sera in the detection of the source of blood. Here 

 it is necessary to use a high dilution of the blood to be tested, and 

 if a reaction is given, to try again with higher dilutions until the 

 limit is reached. In testing dried blood-stains for medico-legal 

 purposes it is, of course, usually impossible to determine the exact 

 amount of serum which has been taken up by the solvent (normal 

 saline solution). It is found, however, that a i : 1,000 dilution 

 of serum in normal saline solution will give a good froth when air 

 is allowed to bubble through them from a pipette, and this will 

 give a rough idea as to the amount of proteid material dissolved 

 out of the clot to be tested. 



The delicacy of the reaction is truly astonishing. Thus Ascoli 

 obtained an anti-egg albumin serum which gave a precipitate with 

 a i : 1,000,000 dilution of egg-albumin,- and Stern an antihuman 

 serum which reacted with serum at a dilution of i : 50,000. These 

 are extreme figures, but sera active on solutions diluted i : 5,000 are 

 frequently obtained. 



As regards the substances for which precipitins can be obtained, 



