THE PHENOMENON OF PRECIPITATION 



253 



dilution of the protein which was injected, turbidity and rapid floccu- 

 lation will result. In tests of this kind, unlike the bacterial precipitin 

 tests in which the delicacy of the reaction is ordinarily determined 

 by diminution of the amounts of antiserum, the same object may be 

 more conveniently attained by dilution of the antigen. Thus, in test- 

 ing the precipitating potency of, let us say, the serum of a rabbit 

 immunized with sheep serum, we would proceed by setting up a 

 series of small tubes, each of which contains a constant amount of 

 antiserum (precipitin), but a progressively diminishing amount of 

 antigen in the same volume i. e., in dilution with isotonic salt solu- 

 tion. The following example will make this clear : 



In this test it will be noticed that the strongest concentration of 

 the antigen (1:10) gave a relatively slight precipitation only. This 

 phenomenon is analogous to the inhibition zones noticed in the case 

 of agglutination and other antibody reactions and will be more espe- 

 cially discussed in a succeeding paragraph. 



The delicacy of the above example, moreover, is by no means 

 unusual, and sera have been obtained by careful immunization with 

 which the specific antigen could be detected in dilutions as high as 1 

 to 100,000 (Uhlenhuth). A serum which will react with antigen 

 dilutions of 1 to 10,000 and 1 to 20,000 is not at all uncommon nor 

 difficult to obtain. Apart from the additional advantage of the 

 specificity of the reaction, therefore, this biological method of de- 

 tecting proteins is more delicate than that of any of the known 

 chemical methods; neither the Biuret nor Millon's reaction will far 

 exceed a delicacy of 1 to 1,000. By a modification known as the 

 method of Complement or Alexin-fixation, furthermore, the delicacy 

 of the biological reactions can be still further enhanced. This is 

 discussed in detail in another place (see page 212). 



The practical value of the precipitin reaction, however, lies, not 

 in the mere detection of protein, but, by virtue of its specificity, 22 in 

 the determination of the variety of protein under examination. And 



22 Wassermann and Schiitze. Deutsche med. Woch., 1900, Vereinsbeilage, 

 p. 178; Berl. kl. Woch., 1901; Deutsche med. Woch., 1902; Bordet, Ann. Past., 

 Vol. 13, 1899; Nolf, ibid., Vol. 14, 1900; Fish, Medical Courier, St. Louis, 

 1900, cited from Wassermann. 



