Initial recognition of microbes, as they enter the body, is based on germ line-encoded pattern recognition receptors (PRR) that selectively bind to essential components of pathogens, so-called pathogen-associated molecular patterns (PAMP). Some of the most important PRR are the TLRs, 10 of which have been recognized in humans: TLR1, TLR2, ... TLR10 (a TLR11 has been found in mice). The TLRs are type I integral membrane glycoproteins and members of a large so-called Toll/IL-1R superfamily of signal molecules.The natural ligands of the TLRs are evolutionary conserved microbial product Those recognizing bacterial and fungal PAMP are generally displayed on the surface of cells, and those recognizing viral components are found intracellularly:

The triggering of various TLRs, or sets of TLRs, activates different intracellular signaling pathways resulting in different repertoires of genes being activated (different repertoires of cytokine genes are depicted above). This means that the innate immune system allows cells to react with an immediate and balanced response to infection depending on the infectious organism: