ANTIGENS, HAPTENS, ANTIBODIES, ETC. 401 



J>e altered in various ways with corresponding changes in 

 specificit}'. Aoetylation, which involves the hydroxyl 

 groups of tyrosine as well as amino groups, has a greater 

 determining influence on specificity than does treatment 

 with formaldehyde which acts primarily on amino groups. 

 Acetylation usually eliminates species specificity but the 

 action of formaldehyde does not. It may be mentioned 

 here that the chemical alteration of a protein from a 

 given animal may change it sufficiently to cause it to 

 react as a protein foreign to that animal ; thus rabbit 

 serum treated with formaldehyde is sufficiently different 

 from the original serum to elicit the production of anti- 

 bodies when it is injected into the rabbit which supplied it. 



Polysaccharides. — The majority of polysaccharides, 

 bacterial and otherwise, which have been examined are 

 not antigenic although they are haptens, that is, they 

 can react with antisera prepared against a complete 

 antigen of w^hich they formed a part. It was shown by 

 Zozaya, that starch, dextran, glycogen and the poly- 

 saccharides of several bacteria, including B. anthracis, 

 the dysentery bacilli, streptococcus and pneumococcus, 

 became antigenic if they were adsorbed on to collodion 

 or aluminium hydroxide as a colloidal carrier. Since 

 that time several polysaccharides have been suspected of 

 being in themselves antigenic, without requiring any 

 colloidal carrier, in spite of being free from proteins. The 

 first of these was the acetyl polysaccharide isolated from 

 the Type I pneumococcus by Goebel. This substance 

 differs from the originally isolated soluble specific sub- 

 stance in the possession of one acetyl group, which is 

 apparently sufficient to convert the hajiten into a com- 

 plete antigen. Some doubt has been cast on this finding 

 by the work of Felton who could find no correlation 

 between the acetyl content of various samples and their 

 antigenicity. 



The next supposedly antigenic polysaccharide to be 



