TLR4 and acute respiratory distress syndrome: LPS, the main pathogen-associated molecular pattern inducing ALI, interacts with TLR4 receptor on the effector cytomembrane to trigger an intracellular signal transduction system, which can activate the TLR4/NF-κB pathway to initiate gene transcription and produce various pro-inflammatory cytokines, such as IL-1β, IL-6, and TNF-α.