PART III. 



BIOLOGICAL METHODS FOR THE IDENTIFICATION 

 AND DIFFERENTIATION OF PROTEINS. 1 



SECTION XXII. THE PRECIPITIN REACTION. 



WHEN the necessity arises for differentiating between nearly allied 

 proteins of different origins the ordinary chemical and physical 

 methods entirely fail. No reliable chemical methods exist for dis- 

 tinguishing, for example, between human blood and the blood of 

 other species, or between the muscular tissue of one animal and the 

 muscular tissue of another. Yet, in actual practice, both in forensic 

 medicine and in the ordinary routine of food -inspection and analysis, 

 the necessity for determining differences of this description frequently 

 arises. For this purpose biological methods, and especially the so- 

 called precipitin reaction, have hitherto been almost exclusively 

 employed. 



The first observations dealing with this subject date from 1 897 and 

 are due to Kraus, who showed that by the injection of typhus bacilli 

 into an animal a serum was produced, which not only caused ag- 

 glutination of the bacteria, but also produced a precipitate with the 

 filtrate of the culture medium. The reaction was found to be specific 

 for certain substances contained in this medium. 



In 1899 Bordet and Tschtistowitsch obtained quite similar re- 

 sults with animal cells and cell products. By the injection of horse- 

 serum, eel-serum, cows' milk, etc., into rabbits, sera could be obtained, 

 containing the so-called precipitin, which gave thick precipitates 

 with the substances used to produce them, and with these substances 

 (the so-called precipitinogens) only. A relatively simple biological 

 method was, therefore, available for distinction between proteins 

 from various sources. 



A large number of investigations followed the observations of 

 Bordet, which were undertaken with the main object of determining 

 how far the reactions were distinctly specific. 



Bordet himself showed that the serum of a rabbit, immunised 

 against cows' milk, produced a precipitate with this milk and not with 

 that of a goat, and Wassermann and Schiitze, Uhlenhuth and others 

 found a similar specificity for precipitins produced by egg-white 

 and blood. Of special interest are the blood precipitins. If human 

 blood be injected into a rabbit a serum is produced which gives a 



1 There is a very large literature on this subject, which can be only very briefly dealt 

 with in this place. Excellent summaries are given together with the principal literature 

 references in the papers of L. Blum and G. Blume. Reference should also be made to 

 the exhaustive monograph on the blood test by Nuttall. 



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