56 Chemical Basis of Genus and Species 



called forth by the blood of the immunized animal is 

 specific, inasmuch as the proteins from a hen's egg 

 will call forth the formation of precipitins in the blood 

 of the rabbit which will precipitate only the white of 

 egg of the hen or of closely related birds. 



To Nuttall 1 belongs the credit of having worked out 

 a quantitative method for measuring the amount of 

 precipitate formed, and in this way he made it possible 

 to draw more valid conclusions concerning the degree 

 of specificity of the precipitin reaction. He found 

 by this method that when the immune serum is 

 mixed with the serum or the protein solution used for 

 the immunization a maximum precipitate is formed, 

 but if it is mixed with the serum of related forms a 

 quantitatively smaller precipitate is produced. In 

 this way the degree of blood relationship could be 

 ascertained. He thus was able to show that when the 

 blood of one species, e. g., the human, was injected into 

 the blood of a rabbit, after some time the serum of the 

 rabbit was able to cause a precipitate not only with the 

 serum of man, or chimpanzee, but also of some lower 

 monkeys; with this difference, however, that the pre- 

 cipitate was much heavier when the immune serum was 

 added to the serum of man. The method thus shows 

 the existence of not an absolute but of a strong quanti- 

 tative specificity of blood serum. This statement may 



1 Nuttall, George H. F., Blood Immunity and Blood Relationship, 

 Cambridge Univ. Press, 1904. 



