Participates in the innate immune response to microbial agents. Acts via MYD88 and TRAF6, leading to NF-kappa-B activation, cytokine secretion and the inflammatory response (By similarity). Binds MYD88 via their respective TIR domains (By similarity). Homodimer (Potential). Highly expressed in spleen, lymph node, thymus, tonsil and at lower levels in lung. Highly expressed in promyelocytic HL-60 cells and in B-cell lines. Belongs to the Toll-like receptor family. In some plant proteins and in human SARM1, the TIR domain has NAD(+) hydrolase (NADase) activity (By similarity). However, despite the presence of the catalytic Asp residue, the isolated TIR domain of human TLR4 lacks NADase activity (By similarity). Based on this, it is unlikely that Toll-like receptors have NADase activity.