Toll-like receptors (TLRs) are type I transmembrane proteins.
They contain an extracellular leucine-rich repeat (LRR) domain that
recognizes conserved pathogen-associated molecular patterns
(PAMPs), a transmembrane domain, and an intracellular Toll/IL-1
receptor (TIR) domain that activates downstream signaling pathways
[1]. TLRs play an essential role in the activation of innate
immunity by recognizing their cognate ligands, such as lipoproteins,
double-stranded RNA, single-stranded RNA, lipopolysaccharide
(LPS), flagellin, and genomic DNA [2].