a type of molecule which is a major component of the cell wall of Gram negative bacteria; an important antigen.

Lipid-containing compounds; lipopolysaccharides are endotoxins and important group-specific antigens. They are often derived from the cell wall of gram-negative bacteria and induce immunoglobulin secretion. The lipopolysaccharide molecule consists of three parts: lipid a, core polysaccharide, and o-specific chains (o antigens). Source: http://www.biology-online.org/dictionary/Lipopolysaccharides