The Toll-like receptor (TLR) family in mammal comprises a family of transmembrane proteins characterized by multiple copies of leucine rich repeats in the extracellular domain and IL-1 receptor motif in the cytoplasmic domain. Like its counterparts in Drosophila, TLRs signal through adaptor molecules and could constitute an important and unrecognized component of innate immunity in humans. The TLR family is a phylogenetically conserved mediator of innate immunity that is essential for microbial recognition. TLRs characterized so far activate the MyD88/interleukin-1 receptor-associated kinase (IRAK) signaling pathway. Thirteen homologs of TLRs (TLR1-13) have been described. Toll-like receptor 5 (TLR5) expression is upregulated following exposure to bacteria or to the TLR5 agonist, flagellin. Gram-negative bacteria, stimulate monocyte/macrophage cells in a TLR5-specific, CD14-independent manner. The TLR5 receptor thus appears to be the principal means by which the innate immune system recognizes flagellated bacterial pathogens.
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