Toll-like receptor signaling

Toll-like receptors (TLRs) play essential roles in innate immune responses. The name TLR is derived from a Drosophila protein, Toll, which detects fungal infection in the fruit fly. The immune system in Drosophila is entirely dependent on a limited number of germline-encoded receptors for pathogen recognition. In contrast, the vertebrate immune system is characterized by the evolution of acquired immunity in addition to innate immunity. Acquired immunity is mediated by T and B cells, which utilize rearranged receptors. This system is advantageous for detecting pathogens with high specificity, eradicating infection in the late stages and establishing an immunological memory. However, the mammalian innate immune system plays critical roles in the initial defense against invading pathogens and subsequent activation of the acquired immune system. Innate immune cells, such as macrophages and dendritic cells (DCs), sense pathogens through TLRs, phagocytose them and evoke immune responses.