Recognition of Gram-negative bacteria and endotoxin by the innate immune system

Until about 10 years ago the exact mechanisms controlling cellular response s to the endotoxin - or lipopolysaccharide (LPS) - of Gram-negative bacteri a were unknown. Now a considerable body of evidence supports a model where LPS or LPS-containing particles (including intact bacteria) form complexes with a serum protein known as LPS-binding protein; the LPS in this complex is subsequently transferred to another protein which binds LPS, CD14. The l atter is found on the plasma membrane of most cell types of the myeloid lin eage as well as in the serum in its soluble form; LPS binding to these two forms of CD14 results in the activation of cell types of myeloid and nonmye loid lineages, respectively.