Toll-like receptors (TLRs)
The initial recognition of microbes during host defense is mediated by a family of receptors termed Toll-like-receptors (TLRs) [1]. Activation of the TLRs leads not only to the induction of inflammatory responses, but also to the development of antigen-specific innate and adaptive immunity [2, 3]. TLRs recognize conserved microbial components, termed pathogen-associated molecular patterns (PAMPs). To date, 12 TLRs have been cloned in mice and 10 in human and each receptor appears to be involved in the recognition of a unique set of PAMPs that are distinct in their chemical nature and structure [4]. TLRs can be divided into two categories with regard to their subcellular localisation. Cell surface TLRs includes TLR1/TLR2, TLR4 /MD2, TLR5 and TLR6 /TLR2 whereas TLR3, TLR7, TLR8 and TLR9 are found in intracellular organelles [5]. Toll-like receptors are characterized by an extracellular domain composed of leucine-rich repeats (LRRs), and an intracytoplasmic domain with a conserved region found in TLRs as well as in Interleukin-1 receptor (Toll/IL-1R or TIR domain). TLR intracellular domains specifically recruit several adaptor proteins including MyD88, TIRAP/MAL, TRIF, and TOLLIP[6]. These adaptor proteins subsequently associate with a family of Interleukin-1 Receptor-Associated Kinases (IRAK1, 2, M, and 4). Recruitments of numerous downstream signaling proteins lead to activation of a range of transcription factors such as NF-κB, AP-1, and IRFs, which are responsible for specific gene transcription [7]. 

TLR ligands
Biochemical studies and genetic analyses using transgenic mice have revealed specific ligands for the activation of the TLR receptors. Of the 12 mouse TLRs (1 to 9 and 11 to 13) and 10 human TLRs described to date, only human TLR 10 and mouse TLR 12 and 13 are orphan receptors. TLR1, TLR2 and TLR6 (both as homo- and heterodimers) detect lipopeptide [4], while TLR3, TLR7, TLR8 and TLR9 recognize nucleic acids [5]. TLR5 senses Flagellin, a protein found in the flagella of gram-negative bacteria [8] and TLR4 recognizes a diverse collection of lipopolysaccharides (LPS) [9]. The extracellular region including the LRRs either directly or indirectly binds to their respective ligands. For instance, MD2, CD14 anf LPS-binding protein (LBP) are the co-receptors of TLR4, while flagellin, an unmethylated CpG oligonucleotides directly interact with TLR5 ad TLR9, respectively [9]. TLR11 senses the profilin-like protein from the protozoan parasite Toxoplasma gondii [10].