SPECIFICITY 171 



antigen without altering its specificity for species. Appreciating 

 that the number of different aromatic radicals in the protein mole- 

 cule is not sufficient to account for the innumerable manifestations 

 of specificity, Pick interprets the significance of these aromatic 

 radicals as that of a central complex about which are the groupings 

 which determine species specificity.^'-* It is not merely the number 

 and proportion of amino-acid radicals in the protein molecule which 

 determine its specificity, but, more important because presenting 

 greater possibilities for variations, the arrangement of these radicals 

 in the molecule. 



Landsteiner and Lampl^** have also carried on an extensive study 

 of the precipitin reactions of horse serum when combined with azo- 

 compounds, with chlorine and bromine and with sulfonic and arsenic 

 acids. Their results confirm the observations and conclusions of 

 Obermayer and Pick and of Wells and Osborne, that specificity de- 

 pends upon certain groups within the protein molecule. They made 

 the interesting observation that if one derivative of a protein reacted 

 with another sort of derivative, the position in the molecule of the 

 substituted radicals was identical or closely related. That is, cross 

 reactions depend on chemical relationships, as Wells and Osborne also 

 found by means of the anaphylaxis reactions, and the specificity is 

 determined by relatively small portions of the large antigen molecule. 

 The observation that the location in the molecule of definite groups is 

 indicated by their immunological reactions can best be explained as 

 depending on spatial correspondence of antigen and antibody, just 

 as Emil Fischer assumed for the specific action of ferments in his 

 comparison to "lock and key." Here again we get evidence that 

 both chemical composition and spatial relations are concerned in deter- 

 mining specificity. Presumably there are also innumerable isomeres 

 that cannot be distinguished by our present methods, which correspond 

 to the racial and individual differences which are so obvious and yet 

 not to be detected by serum reactions. 



Contemplating the possible number of variations in the arrange- 

 ment of the amino-acids in a protein which the great number of these 

 radicals provides, there is no diflB.culty in understanding the existence 

 of an almost limitless number of specific distinctions between proteins. 

 Abderhalden, indeed, calculates that the 20 amino-acids we find in 

 proteins could form at least 2,432,902,008,176,640,000 different com- 

 pounds, and this without including possible compounds varying in 

 quantitative relations. A contribution to the chemical basis of speci- 

 ficity has been made by Kossel,''^ who finds certain relations in the 



''Landsteiner and Prasek (Zeit, Immunitat., 1913 (20), 211), however, state 

 that alteration of proteins by simply treating them with acid alcohol also causes 

 them to lose their species specificity, and this without anj'- substitution in the 

 aromatic radicals of the proteins. This observation throws doubt on the hypothe- 

 sis of Pick that the aromatic radicals are the essential center of species specificity. 



«» Biochem. Zeit., 1918 (86), 343. 



«i Zeit. physiol. Chem., 1913 (88), 163. 



