...a single phylogenetically conserved domain, called Lysin (LysM), is found in specific receptors in signaling pathways responsive to one or more of these three related carbohydrates in bacteria, plants, and fungi and possibly mammalian systems. ...promiscuous activation could occur when a structurally similar but physiologically inappropriate ligand binds and thereby aberrantly activates an incorrect LysM domain-containing receptor. In support of this idea, Nod factors suppress an innate immune reaction in Arabidopsis thaliana, even though this species lacks a Nod receptor 14. Since this effect depends on the presence of the LYK3 LysM-containing receptor kinase that functions as an innate immune receptor, the Nod factor could therefore act as a competitive antagonist of LYK3. Specifically, peptidoglycan is released by growing bacterial cells, and the substantial levels of peptidoglycan fragments generated by the microbiota have important systemic immunological effects mediated by key innate immune proteins like Nod1 30. ...perhaps the levels of these molecules relative to their receptors is affected by the presence of chitin, which changes the normal ligand:receptor stoichiometry. ...inflammatory bowel disease is another example in which interactions between chitin- and peptidoglycan-signaling systems could be relevant.