Abstract: Toll, a protein involved in the dorsoventral development of fruit flies, was first discovered by the German Scientist Christiane Nüsslein-Volhard and his colleagues in 1985. In 1996, Jules Hoffmann at the National Center for Scientific Research in Strasbourg, France, discovered that the product of Toll was involved in  the sensing  of pathogenic infections in fruit flies, and its activation was necessary for the initiation of effective defense; In 1998, Bruce A. Beutler at the Scripps Research Institute  discovered that the lipopolysaccharide (LPS, a microbial pathogenic product) resistant mouse had a mutation in the receptor gene very similar to the Toll in fruit flies, and proved the Toll-like receptor was the sensor of LPS. These discoveries showed that mammals and fruit flies adopt the similar molecules to activate innate immunity. Therefore, they shared the half of the 2011 Nobel Prize in Physiology or Medicine for their discoveries concerning the sensors for the activation of innate immunity. Another half of the prize was awarded to Ralph M. Steinman at the Rockefeller University in New York. In 1973, Steinman discovered the Dendritic cells(DCs) and proved that it could activate the T cells and initiate the adaptive immune response. Further studies showed that the DCs could sense the signals arising from the innate immune response and control the T cell activation. This makes it possible for the immune system to react towards pathogenic microorganisms while avoiding attacks on own body's endogenous molecules. These discoveries provide us further insights into the mechanism of activation and regulation of the immune system, and will facilitate the development of novel methods in disease prevention and treatment.