ANTIGENS, HAPTENS, ANTIBODIES, ETC. 415 



coupling the azide of N-carbobenzoxy-3 : 5-di-iodothyro- 



nine, 



I 



H0<^ \-0-( \cH2.CH.COX3 



^ NH.OC.OC.H; 



with proteins in alkaline solution and iodinating the 

 product to convert the di-iodothyronine residues to thy- 

 roxine and to convert the tyrosine residues initially in 

 the protein to di-iodotyrosine. Antisera prepared hy the 

 injection of these antigens were highly specific in their 

 reactions and were able to prevent the metabolic activity 

 of thyroglobulin or of thyroxine when these were injected 

 into animals. Similar results were obtained by coupling 

 aspirin to proteins ; passive immunisation of animals 

 with antisera prepared against aspirin antigen prevented 

 the ordinary pharmacological action of aspirin. 



A method of affecting the specificity of proteins which 

 does not necessarily affect the benzene nuclei is to substi- 

 tute the hydroxyl and amino groups of the amino -acids 

 by acyl groups"! A certain amount of cross -reaction 

 between different acyl proteins occurs ; for instance, 

 acetyl-proteins react to some extent with antisera prepared 

 against propionyl-proteins but not with those prepared 

 against proteins containing longer chain substituents like 

 butyryl, CH3(CH2)oCO— , or valeryl, CH3(CH2)3CO— . 

 Proteins containing aromatic substituents like the anisoyl 



CHgO/^ ^CO— , or cinnamyl, <^ \cH=CH.CO— groups 



do not give cross -reactions with antisera to the aliphatic 

 derivatives. Methyl or similar alkyl groups can be intro- 

 duced into the hydroxyl or amino groups of the amino - 

 acids with similar but weaker effects, a result which is to be 

 expected in view of the less actively polar character of 

 such groups. Methylated proteins react with the antisera 

 to ethyl-proteins but not with those to untreated proteins 



