Innate immunity is our first defense against invaders. The epithelial and mucosal linings of our respiratory and GI tracts, as well as our skin are major components of the innate immune system. These act as physical and chemical barriers against pathogens. Cells known as macrophages also play a large role in innate immunity. Macrophages recognize bacterial or viral components such as lipopolysaccharide (LPS) or double-stranded RNA (dsRNA), via special receptors known as Toll-like receptors (TLRs). TLR activation cause macrophages to secrete cytokines (small molecules involved in cell signaling and attraction), as well as to phagocytose the infected cells. The innate immune system is required to activate our adaptive immune system.