Endotoxin detection took significance early on protein expression work, as the main expression vehicles were of bacterial origin. Endotoxins are lipopolysaccharides (LPS) present in the outer membrane of the cell wall of Gram negative bacteria. Toxicity is associated with the lipid component (Lipid A) and immunogenicity is associated with the polysaccharide components. The cell wall antigens (O antigens) of Gram-negative bacteria are components of LPS. LPS elicits a variety of inflammatory responses in mammals and it activates complement by the alternative (properdin) pathway, so it may be a part of the pathology of Gram-negative bacterial infections.
In 1986 a plasma protein named LPS binding protein (LBP) helped elucidate the mechanism by which the LPS-LBP complex binds to CD14 receptors on monocytes and macrophages. Subsequent
interaction with Toll-like receptor 4(TLR4) results in the release of cytokines(IL-1,6 and 8), TNF and platelet activating factor; all of which cause the secretion of prostaglandins, activation of the complement and coagulation cascades; ultimately all this leads to acute inflammation, hemorrhage and septic shock. It is clear then, that any in vivo experiment can be affected by the presence of undesired endotoxins.