The previously characterized TLR ligands, for example, LPS, lipoteichoic acid and peptidoglycan, have all of these characteristics. Whether the same is true for bacterial DNA is not entirely clear.

Each TLR can be activated by a variety of molecules; however, TLR4 mainly recognizes LPS, TLR2 mainly recognizes MDP, TLR5 mainly recognizes flagellin, while intracellularly expressed TLR9 mainly ...