The human body is a complex machine, and when it suffers from an attack on its system by pathogenic microbes such as bacteria, it acts accordingly to find and eliminate the source of the infection and restore function back to normal. The way in which it protects itself from infection, is through the immune system, but how it is able to locate infectious microbes had managed to elude science until recently, with the discovery of Toll-like receptors.
Toll-like receptors are a set of proteins that form part of the innate immune system which dictates how the adaptive immune system works. These receptors were first thought to be involved in infection in 1996, when Jules Hoffmann was able to show that the Toll receptor in Drosophila flies, of which Toll-like receptors are named after, was involved in the fly’s immune system being able to detect fungal and bacterial infections. With the discovery that a Toll-like receptor called TLR4 in humans could be artificially stimulated to initiate an immune response in 1997, Bruce Beutler discovered that TLR4 was involved in sensing the bacterial toxin lipopolysaccharide and thus acted as a sensor for bacterial infection.
With the discovery of the function of Toll-like receptors in humans as immune response initiators, it has since been found that there are 10 known Toll-like receptors to date, with another 3 present in mice but not humans. The ability of the human body to fight off infection is dependent on these receptors, and each of them bind to specific microbial molecules that are unlike molecules in the host organism and thus the organism is able to detect infections without harming itself.
When an infection is present, the innate immune system is alerted to its presence through the Toll-like receptors binding to a microbial molecule, such as through the TLR4 receptor binding to the endotoxin lipopolysaccharide in the presence of bacterial infection. This leads to the activation of other immune functions based on the type of infection, which fight it off. In the case of a bacterial infection, cytokines that trigger inflammation may be released, and the bacteria is engulfed by phagocytes which destroy the bacteria through enzymes or oxidisation.
Toll-like receptors are immensely important to the function of the immune system. There are still factors behind their the immune system as a whole the are not properly understood, but with the discovery of Toll-like receptors, drugs that act on these receptors, such as imiquimod used in dermatology, are being developed to enhance and control the immune response and fight off infection, improving the current medicine available to treat dangerous infections.