A family of receptors found in mammals that are thought to be involved in the recognition of pathogens by the immune system.

Named for their sequence homology to the Drosophila melanogaster receptor Toll.

As of January 2001, 10 Toll-like Receptors (TLRs) have been discovered.

Stimulation of Toll-like Receptors by bacterial components PAMPs lead to, among other things, TNF-alpha release and macrophage activation.

Here are what each receptor is thought to recognize (as of October 29, 2001):

• TLR1 - works as a heterodimer with TLR2 to recognize tri-acylated lipoproteins
• TLR2 - works as a heterodimer with TLR1 or TLR6 to recognize peptidoglycan, lipopeptides, yeast cell walls, variant LPS
• TLR3 - double stranded RNA
• TLR4 - LPS, β-defensin 2, heat shock proteins (this last point is debatable, and a recent study suggests that the finding was an experimental artifact)
• TLR5 - flagellin (I discovered this fact)
• TLR6 - works as a heterodimer with TLR2 to recognize di-acylated lipoproteins, peptidoglycan, yeast cell walls
• TLR7 - imidazoquinoline compounds including Imiquimod, a anti-viral substance used to treat warts; and Resiquimod, a variant from another pharmaceutical company. These are artificial compounds, so it is highly unlikely that TLR7 evolved to specifically recognize these compounds. The 'natural' ligand for TLR7 has not yet been identified.
• TLR8 - surprisingly, the same as TLR7. Mice lack functional TLR8
• TLR9 - bacterial DNA