TLR4 and Anxiety: Studies in humans have previously associated intestinal permeability with increased anxiety and depressed mood states, hypothesized to occur through low-grade exposure to bacterial antigens (e.g. LPS) that are capable of activating toll-like receptor 4 (TLR4), leading to initiation of signal transduction pathways responsible for innate and acquired immune response (Galland, 2014; Kelly et al., 2015).