402 BACTERIOLOGICAL CHEMISTRY 



isolated was the "complete antigen" which Boivin and 

 his co-workers isolated from smooth strains of Salmonella 

 typhirnurium by extraction with dilute trichloracetic 

 acid. It was found to be toxic, capable of provoking 

 antibody production and of immunising mice against 

 subsequent injection of many times the fatal dose of the 

 toxin or the living organisms. By the action of hot, 

 dilute acetic acid the antigenic character and toxicity 

 were rapidly lost and a polysaccharide, the " residual 

 antigen," was obtained which was a hapten only, the 

 hydrolysis having split off fatty acids and a phosphatide. 

 It has since been shown that a protein fraction is also 

 removed by hydrolysis and that the antigenicity of the 

 " complete antigen " depends on its presence (see p 361). 

 Rough strains of Sal. typJiimurium contain the " residual 

 antigen " only ; they possess an enzyme which hydro lyses 

 the " complete antigen." The same antigen was dis- 

 covered almost simultaneously by Raistrick and Topley, 

 who isolated it by treating the bacteria with trypsin in 

 order to digest the bacterial protein and then precipitating 

 the polysaccharide from solution with alcohol. 



More recently, in a similar way, Raistrick and Topley 

 have isolated a toxic, completely antigenic polysaccharide 

 from the typhoid bacillus . It confers protective immunity 

 on mice when injected in extremely small doses and elicits 

 antibody formation in rabbits. On hydrolysis by very 

 weak acid this too loses acid groups and yields a neutral 

 polysaccharide which is non-toxic and non-antigenic (see 

 p. 363). 



It is, therefore, becoming more certain that pure 

 polysaccharides are not antigenic, but that when com- 

 bined with a protein they act as the determinant groups 

 of an antigenic complex. 



Lipoids. — There is still considerable controversy as to 

 the antigenic character of lipoids, although there is no 

 doubt of the very important part which they play in 



