74 THE GENERAL CHARACTERS OF THE PROTEINS 



The precipitin is generally prepared by several injections, gene- 

 rally intraperitoneal, but sometimes subcutaneous or intravenous, 

 following one another at intervals of from three to six days. 1 Rabbits 

 are the animals commonly employed. The more precipitin a serum 

 contains the less specific is it, z>., the more readily will it precipitate 

 proteins other than the precipitinogen. For practical purposes, there- 

 fore, it is not advisable to employ precipitins of very high grade ; if 

 sera be obtained which give precipitates with bodies other than the 

 precipitinogens, it is advisable to dilute them before use. In de- 

 termining the origin of a sample of blood the material to be in- 

 vestigated (clothes, etc.) is extracted with physiological saline, and 

 the extract is filtered through a Berkefeld filter and diluted, so that a 

 solution containing O'l per cent, protein is obtained. To 2 c.c. of 

 such a solution O'l c.c. of the precipitin-containing serum is added. 

 The more nearly the protein in the material under investigation is 

 allied to the precipitinogen employed for the preparation of the 

 antiserum, the greater the dilution in which a precipitate will 

 appear. To determine, therefore, the origin of a given sample of 

 blood (e.g., human blood), samples of other bloods should be used as 

 controls ; the precipitin prepared by immunising a rabbit against 

 human blood, for example, will give with the material under investi- 

 gation a precipitate in much greater dilution, should it contain human 

 blood, than it would if it contain blood from any other species. 

 Furthermore, the more nearly allied the species to that from which 

 the precipitinogen has been derived, the more readily will its protein 

 give a precipitate with the precipitin. This reaction has been ex- 

 tensively employed by Nuttall for determining the genetic relation- 

 ships of different species. 



Another method of applying the precipitin reaction has been 

 recently introduced by Neisser and Sachs. When haemolysis of red 

 blood corpuscles is brought about by a serum the latter contains 

 two different bodies, both of which are necessary for the process 

 viz., the heat-labile complement and the heat-stable amboceptor. 

 Gengou has shown that, when a precipitate is formed by bringing 

 together precipitin and precipitinogen in the presence of a haemolytic 

 serum, the complement disappears, even when the amount of pre- 

 cipitate is so small as to be hardly visible. A haemolytic serum 

 can be tested as regards its haemolytic power towards a given 

 suspension of red blood corpuscles. To a similar quantity of the 

 same serum may be added a precipitin-containing serum which is 

 not haemolytic towards the same corpuscles. If to such a mixture a 

 protein be added containing a substance which will form a precip- 

 itate with the precipitin, it will lose its haemolytic properties. In 

 this way Neisser and Sachs have succeeded in detecting human 

 blood in dilutions of I in 10,000, or even I in 100,000. For sug- 

 gested explanations of these phenomena reference must be made to 

 the original papers. 



To illustrate the method of employment of the precipitin reaction 

 a short description of Schulz's method for the quantitative estima- 

 tion of proteins in mixtures is appended. 



1 Nuttall generally used 5-10 c.c. of serum for each injection, but in some cases 

 smaller quantities. For details see his Monograph, pp. 54, 55. 



