Toll-like receptors (TLRs) are a type of protein found on the surface of certain immune cells. They play a crucial role in the body's defense system by recognizing pathogens like bacteria and viruses. When TLRs detect these harmful invaders, they trigger an immune response, helping the body to fight off infections. TLRs are part of the innate immune system, which is the body's first line of defense. They are named after the Toll gene discovered in Drosophila, a type of fruit fly. This discovery has led to significant insights into how the immune system functions in humans and other animals.