Comparative studies on the immunologic properties of free endotoxin and lipopolysaccharide from Shigella sonnei.

Immunogenic, antigenic and toxic properties of free endotoxin isolated from culture media of Shigella sonnei, phase I, was studied. Experiments with mice showed that free endotoxin is about 3--4 times more active immunogenically and twice as toxic as cellular lipopolysaccharide. On the other hand, the antigenic activity of free endotoxin was about equal to that of lipopolysaccharide. In addition, free endotoxin had a common property characteristic for endotoxins isolated from Gram-negative bacteria, namely it stimulated the immunologic response to unrelated antigens, e.g. antigens of sheep red blood cells.