Toll-like receptors are type I transmembrane signaling receptors. They are primordial pathogen-recognition proteins that function as sentinels for the innate immune system. TLR4, also known as CD284, is a 110kD protein which is expressed on monocytes/macrophages, endothelial cells, and at low levels on B cells and granulocytes. In association with a secretory molecule, MD2, TLR4 has been recognized as critical for host recognition of bacterial LPS.