346 IMMUNOLOGY 



antiserum for each produced the maxmium amount of precipi- 

 tate with its homologous antigen, but the anti-egg-white-diazo- 

 tized-arsanilie-acid protein gave weaker reactions with diazo- 

 tized-arsanilic-acid-gelatin antigen and conversely the latter anti- 

 serum gave weaker reactions with the former. 



Egg Albumen Contains Tyrosine and Histidine. Gelatin Con- 

 tains HiSTiDiNE. — It occurred to them that egg white contained 

 both tyrosine and histidine, while gelatin contained histidine but 

 no tyrosine. Hence egg white would have at least two amino acids 

 linked to the arsanilic acid while gelatin would have only one; 

 i.e., histidine. These are the two amino acids which couple with 

 the diazonium compounds. When these antigens are injected one 

 would expect two antihaptens to be developed for the diazotized 

 arsanilic acid egg white and one for the diazotized arsanilic acid 

 gelatin antigen. To determine whether this assumption was true 

 they made use of the phenomenon of suppression of a specific 

 antigen-antibody reaction by compounds chemically similar to tlie 

 specific hapten on the antigen. 



Use of Diazo Dyes for Suppression of Specific Reaction. — It 

 will be remembered that Wormall showed that if he added diiodo- 

 tyrosine to tubes containing antiserum for iodized protein (con- 

 taining tyrosine) and its antigen, the specific precipitate was 

 suppressed. In other words the addition of a hapten to a tube 

 containing the hapten-protein antigen and its specific immune 

 serum results in this case in the suppression of the precipitate. 

 Hooker and Boyd prepared two dyes, one containing tyrosine- 

 likc diazoarsanilic groups and a second containing histidine-like 

 imidazoldiazoarsanilic groups. 



Tyrosinediazoarsanilic Groups in Protein Molecule. — In the 

 protein molecule the tyrosinediazoarsanilic groups are tliought 

 to be as follows : 



