High Quality Content by WIKIPEDIA articles! Lipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS, also called endotoxin) molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane. While its toxic effects can be damaging, the sensing of lipid A by the human immune system may also be critical for the onset of immune responses to Gram-negative infection, and for the subsequent successful fight against the infection.