Lipopolysaccharides, abbreviated LPS, are lipids complexed with a polysaccharide. These are important substances which affect our health, because they are found in the cell walls of gram-negative bacteria. LPS act as endotoxins and group- specific antigens (ie., O antigens). LPa are composed of 3 main parts. 1) Lipid A, a glycolipid which acts as the endotoxin, 2) a core polysaccharide (constant among members of a bacterial strain), and 3) and O-specific chain (variable amongst a bacterial strain). The lipid A portion of the LPS possesses endotoxin activity and this can make a person very ill.