Endotoxin is a type of lipopolysaccharide (LPS), consisting of lipid A (usually 6 acyl chains attached to a phosphorylated disaccharide), attached to the ‘core’ (a short sugar chain with various modifications), which is attached to the O-antigen (a long linear chain of sugars of variable length). Endotoxin is a major component of the outer membrane of gram-negative bacteria, with lipid A in the membrane and the O-antigen constituting the outer-facing surface of the bacterium. Soluble endotoxin is released when bacteria are destroyed, but is also released physiologically as outer membrane vesicles.