The innate immune system is fundamental to the recognition of microbial components with utilizing a series of the innate immune receptors, which triggers the activation of the entire immune system including the acquired immunity. The receptors recognize the molecular patterns of the compounds that are common and unique to the microbes. The microbial cell-surface components are the major ligands of the innate immune receptors such as Toll-like receptors (TLRs) and Nod-like receptors (NLRs). For example, the bacterial glycoconjugates lipopolysaccharide (LPS) and peptidoglycan (PGN) are recognized by TLR4 and Nod1/2, respectively. This chapter summarizes the major classes of the innate immune receptors with their ligands, particularly focusing on the structural features of the TLR4, Nod1, and Nod2 ligands with their immunostimulatory and immunomodulatory activities.
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