98 INFECTION AND RESISTANCE 



react with this species only and the hemolysins, agglutinins, or 

 precipitins produced by the injection of bacterial, cellular, or serum 

 proteins react respectively only with the particular variety em- 

 ployed in their production. This indicates that each of these 

 antigens of almost unlimited number must possess a chemical 

 structure individually characteristic and different from all the others. 

 It is by means of the biological reactions, indeed, that we can detect / 

 protein in dilutions far beyond the reaction-sensitiveness of chemical/ 

 tests and can distinguish between varieties of protein when the chemj/ 

 cal methods will indicate only protein in general. Our knowledge of 

 the chemical constitution of protein has not yet advanced to a point 

 at which specificity can be based upon definite variations of chemical 

 structure, and the complexity of the problem is such that it does not 

 seem likely that we can hope in the near future to attain such knowl- 

 edge. We can merely accept it as a fact that the antibody produced 

 with one protein differs materially from that produced with another, 

 and that this is a definite indication that the antigen in one case must 

 be chemically different from that in another. 



The range of such variations is apparently enormous. For each 

 variety of bacteria or plant, each species of animal, and to a certain 

 extent each individual of the species, possesses certain special anti- 

 genie characteristics peculiar to itself. In general there is an under- 

 lying antigenic similarity which is peculiar to the species. This is 

 true of bacteria and, in the case of animal and vegetable proteins, an ./ 

 antibody produced with material from an individual of a certain 

 species will react with the protein derived from this species in gen- 

 eral. However, that there are also antigenic differences between in- 

 dividuals within the same species is indicated by Ehrlich's experi- 

 ments on the antibodies produced by injecting the blood cells of one 

 goat into another. And we have further indicated that within the 

 same animal different organs may possess individual antigenic char- 

 acteristics. Added to this we know that certain special organs like 

 the testicle, the lens, and some others contain antigens which are 

 peculiar to this variety of organ, irrespective of species a condition 

 spoken of as "organ specificity." Thus an antibody produced by 

 injections of the testicular substance of one animal will react with 

 testicular protein from many different species the specificity here / 

 depending upon the organ and not upon the zoological relationship./ 



It is char, therefore, that there are more different varieties of 

 protein, biologically distinguishable, than there are species of living 

 beings in nature. As Abderhalden 74 has recently pointed out, this 

 is a conception which it is a little difficult to grasp chemically, since 

 in breaking up different proteins into their "building stones" (Bau- 

 steine) we encounter again and again the same 20 amino-acids. By 

 a simple arithmetical consideration, however, he shows that merely 



7 * Abderhalden. Munch, med. Woch., No. 43, 1913. 



