GENERAL CHARACTERS OF LIVING ORGANISMS 35 



effects is to alter constructive metabolism in the cells 

 of the organism in a definite manner so as to give rise 

 to other compounds (apparently also protein) of related 

 or complimentary configuration. These new com- 

 pounds, anti-bodies, form- specific chemical unions with 

 the antigens, and hence may serve as a means of identify- 

 ing the latter or of distinguishing between nearly related 

 proteins, as in the precipitin and anaphylaxis reactions. 

 The living protoplasm responds to the presence of the 

 antigen by synthesizing a compound of similar or 

 complementary configuration; and this chemical resem- 

 blance is what determines the intimacy and specificity 

 of union in the antigen-anti-body reaction. The 

 anaphylactic guinea-pig is in fact the most sensitive 

 means at our disposal for distinguishing between proteins 

 of nearly related composition.' 



It is evident that such phenomena have a most 

 important bearing on the question of the basis of organic 

 specificity. They indicate not only that the synthesis 

 of specific compounds by living protoplasm is determined 

 by the presence of other specific compounds, a fact of 

 general application in the theory of growth processes, 

 but also that the specific syntheses characteristic of a 

 species may be modified under the influence of compounds 

 having a different configuration from those normally 

 present. The indications from precipitin and other 

 tests are that the chemical resemblance between the 

 corresponding proteins of different species is greatest when 

 the biological relationship is closest — when the species are 

 structurally and physiologically most closely similar — 

 and in general decreases as the organic difference increases. 



' Cf. S. Flexner, Science, LII (1920), 615. 



