Toll-like receptors (TLRs) are a family of proteins that play a key role in the innate immune system by recognizing molecular patterns from microbes. They are expressed on immune cells like neutrophils, macrophages, and dendritic cells as well as non-immune cells. Most TLRs are located on the cell surface but TLR9 is intracellular. Stimulation of different TLRs induces distinct patterns of gene expression to activate innate immunity and instruct the development of acquired immunity. The first human TLR was discovered in 1994 and TLRs were shown to induce adaptive immune responses in 1997. TLR signaling involves intracellular adapter proteins and transcription factors that trigger inflammatory responses.