Lipopolysaccharide (LPS) structures may be generally described as three linked regions. The first structural feature is the lipid component Lipid A, primarily responsible for the effects of endotoxin. Linked to Lipid A is the core antigen or R polysaccharide, a short chain of sugars including 2-keto 3-deoxyoctonate (KDO). The third structural feature is attached to the core polysaccharide and is more elaborate, containing up to 40 repeating subunits of 3 to 5 sugars, referred to as the O antigen, O side chain or O polysaccharide. Variations in the composition of the O antigen account for the species specific antibody responses. Lipid A, acting alone or as a component of LPS, is a potent modulator of the mammalian immune response.  The presence of Lipid A or LPS in mammalian macrophages or endothelial cells triggers a signaling cascade leading to the secretion of pro-inflammatory cytokines and nitric oxide. LPS also acts as a B cell mitogen. Differences in Lipid A fatty acid side chain structures may be responsible for variations of the known effects on the immune system.