Any of a family of receptor proteins associated with innate immune responses by lymphocytes, macrophages, and other cells (see immunity). They bind certain constituents of invading pathogens, such as bacterial lipopolysaccharide or peptidoglycan, and activate a signal pathway inside the cell that triggers the release of cytokines and chemokines, such as tumour necrosis factor α‎. TLR activation also leads to the surface expression by dendritic cells and macrophages of co-stimulatory molecules that are essential for activating naive T cells, which can then initiate adaptive immune responses. Toll signalling involves the activation of a transcription factor, nuclear factor κ‎B, which consequently migrates from the cytoplasm to the nucleus, where it activates gene transcription. TLR proteins in humans and other vertebrates are homologous with the Toll protein originally identified because of its role in dorsoventral patterning in embryological development of the fruit fly Drosophila. Similar proteins participate in plant immunity against viruses, which suggests an ancient origin for TLR-based defence mechanisms.