Endotoxins are complex lipopolysaccharides (LPS).LPS has two components,Polysaccharide & lipid A.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 animals. Because it activates complement by the alternative (properdin) pathway, it is often part of the pathology of Gram-negative bacterial infections.
Endotoxins are toxic to most mammals. Even though endotoxins are strong antigens, they seldom elicit immune responses which gives full protection to the animal against secondary challenge with the endotoxin. They cannot be toxoided. Regardless of the bacterial source, all endotoxins produce the same range of biological effects in the animal host.
Compared to the classic exotoxins of bacteria, endotoxins are less potent and less specific in their action, since they do not act enzymatically. Endotoxins are heat stable (boiling for 30 minutes does not destabilize endotoxin), but certain powerful oxidizing agents such as superoxide, peroxide and hypochlorite, degrade them. Endotoxins, although strongly antigenic, cannot be converted to toxoids.
Difference between Endotoxins and Exotoxins are tabulated below
|CHEMICAL NATURE||Lipopolysaccharide (mw = 10kDa)||Protein (mw = 50-1000kDa)|
|RELATIONSHIP TO CELL||Part of outer membrane||Extracellular, diffusible|
|DENATURED BY BOILING||No||Usually|
|POTENCY||Relatively low (>100ug)||Relatively high (1 ug)|
|SPECIFICITY||Low degree||High degree|
|ENZYMATIC ACTIVITY||No||Often |