Effect of proteins on the immunogenicity of enterobacterial common antigen

Enterobacterial common antigen isolated by two independent extraction procedures was found to precipitate with a number of basic or hydrophobic proteins. Complexes of enterobacterial common antigen with protamine sulfate, with methylated bovine serum albumin or with a fraction of outer membrane proteins of two different Shigella wild types proved to be highly immunogenic in rabbits upon intravenous immunization, in contrast to the enterobacterial common antigen preparations by themselves. This explains why crude isolates of enterobacterial common antigen usually are good immunogens in contrast to the isolated antigen, which was described to be either not or only very poorly immunogenic.