420 HUMAN BIOLOGY 



injection of all foreign proteins is alike. But in every case, 

 limited only by the available number of protein antigens, 

 the response is specific for the particular variety injected. 

 Thus the immunological response is so exact differentially 

 that the antibodies elicited by the several proteins vary 

 with every animal and plant species from which the antigens 

 are derived and, in their overlapping, follow with consider- 

 able accuracy zoological and botanical relationships. Anti- 

 bodies to horse serum, for example, react partially with 

 that of zebra, mule and donkey, and the kinship of man 

 with the higher apes may be more fundamentally determined 

 by the similarity of the serum antigens than by any of the 

 more superficial characteristics. 



Of the greatest importance to the biological principles 

 we are discussing are the investigations planned to reveal 

 the properties of the various protein molecules which deter- 

 mine specificity, and which have been authoritatively 

 reviewed by Wells who himself has added much to the 

 understanding of these conditions. There is no longer any 

 question about the fact that immunological specificity is a 

 function of the chemical structure of the particular protein 

 antigen. Immunological similarity is, as Wells has repeatedly 

 shown, based on chemical similarity, while immunological 

 differences are coordinate with chemical differences. 



While the protein nature of the substance as a whole 

 seems essential to its antigenic function, it is nevertheless 

 not the entire molecule which determines the specificity. 

 This has been variously demonstrated by Pick, Landsteiner, 

 Wells and others who have shown that by the introduction 

 of simple radicals, e.g. iodine, diazo and nitro groups which 

 combine with the aromatic ring of certain amino acids, or 

 even by alteration of certain salt-forming groups of the 

 protein, the specificity of the particular protein may be 

 shifted from its original species relationship to another 

 depending on the chemical change. In this way an iodized 

 horse serum produces antibodies that react with other 

 iodized proteins, but not as well, at least, with the original 

 native horse serum. It is quite beyond the scope of this 

 chapter to enlarge upon this most important phase of 

 immunology, and the reader whose interest has been aroused 



