FIGURE 2.

The Toll-like receptor (TLR) family plays a key role in recognizing antigens produced by microorganisms. To date, 13 TLR family members have been discovered. TLR1, 2, 4, 5, and 6 were expressed on the cell surface, while TLR3, 7, 8, and 9 were expressed on the endosome membrane. Toll-like receptors are membrane receptors composed of extracellular domains, single transmembrane helical domains and intracellular signaling domains, which can bind to different ligands (TLR2 and TLR1 or TLR6 complexes recognize lipoproteins or lipopeptides, TLR3 recognizes double-stranded RNA, TLR4 recognizes lipopolysaccharides (LPS), TLR5 recognizes bacterial flagellins, TLR7 or TLR8 recognizes single-stranded RNA, and TLR9 recognizes CpG rich in hypomethylated DNA). When the TLR binds to the respective ligands, different downstream signals can be activated to produce different biological effects.