PRINCIPLES OF PROTEID PRECIPITATION 125 



that of bacterial precipitation. If a clear proteid solution (a) is mixed 

 with the clear serum (a') of an animal immunized against the above 

 proteid (a), turbidity and precipitation will occur; while if a mixture of 

 the serum (a') is made with a non-homologous proteid say (b), or a mixture 

 of the proteid (a) with the serum (b') of an animal immunized against b, 

 no precipitation takes place. Graphically expressed it looks thus: 



a-\-a' = precipitation. 

 b-\-a f = no precipitation. 

 a-\-b f = no precipitation. 

 b + b' = precipitation. 



In other words, a precipitating immune serum reacts only with its homolo- 

 gous proteid. The precipitin reaction is specific. 



It is greatly to the credit of Wassermann and his co-workers 

 Forensic Use ^' Schiitze and Uhlenhuth, who recognized that this speci- 

 of Albumin n ity of precipitins was of great medico-legal value. 

 Differentia- 

 tion. For example, a bloody shirt is found in the home of a man charged with 

 murder; the prosecution sees in that the proof of crime, while the defend- 

 ant pleads that the stains belong to the blood of a sheep; the proof as to 

 their source is of the utmost deciding evidence; and while chemical or microscopical 

 examinations are of little or no use, serum diagnosis wins the day. 



The blood-stained clothing is extracted in water, part of the extract is mixed with 

 a, the serum of a rabbit immunized against human serum and another part is mixed 

 with b, the serum of a rabbit immunized against sheep's serum. If the mixture a 

 shows a precipitate, it can be definitely stated that the blood stain contained serum 

 derived from a human being; while if mixture a is clear and b shows the precipitate, 

 it is strongly corroborative of the presence of sheep's serum. 



This example suffices to indicate the value of this biological 

 Blood fact. In addition the reaction is made use of in the deter- 

 Relationship, mination of the nature of meats (detection of horse meat sub- 

 stitution for beef). 



Furthermore, this method has explained a number of scientifi- 

 cally interesting problems. Just as group agglutination demonstrated the 

 close relationship existing between various bacteria, so also serum precipi- 

 tation proves a distinct relationship between the different species of 

 animals (horse and donkey, dog and fox, hare and rabbit, ape and 

 man, etc.). 



Thus the serum of a rabbit immunized against human serum precipitates not only 

 human serum but also that of monkeys; the serum of a chicken immunized against 

 rabbit's serum precipitates not only that, but also hare's serum. In order, however, to 

 differentiate between rabbit's and hare's serum, Uhlenhuth advises the immunization 

 of a rabbit with hare's serum. The serum of such an immunized rabbit precipitates 



