Endotoxin, toll-like receptor 4, and the afferent limb of innate immunity.

Positional cloning work and subsequent biochemical analyses have revealed that Toll-like receptor 4 (Tlr4) transduces the lipopolysaccharide (LPS) signal, alerting the host to infection by Gram-negative bacteria. Moreover, it appears that the LPS sensing pathway is a solitary one: disruption of Tlr4 causes complete unresponsiveness to LPS. As several Tlr family members exist in vertebrates, it appears likely that the innate immune system defends the host by recognizing a small number of structurally conserved molecules that distinguish the microbial world from tissues of the host.