The third defense system are white blood cells within the bloodstream, which leave the blood vessels and actively seek to destroy foreign pathogens. These white blood cells are also known as phagocytes (Greek definition for ‘to devour cells’), and the most common phagocytes are neutrophils. Neutrophils eat invading pathogens by releasing a chemical compound called HOCL (hypochlorous acid), a process known as phagocytosis. Upon tissue damage, the human body immediately responds to pathogen invasion by emitting histamines, which in turn signals the body to increase blood flow to the site of the infection, allowing neutrophils to exit capillaries at the infected area and hunt for the harmful pathogens to ‘eat’. Neutrophils find the pathogens by following their chemical trails through the process of positive chemotaxis. The neutrophils bind to the outer surfaces of the pathogens for digestion.