Neutrophils have an average diameter of 12-15 micrometers (µm). The average half-life of neutrophils in the circulation is 12 hours when they are not activated. But when they are activated, they live between 1 and 2 days. When a bacterium or a virus first enters the organism, it first encounters a neutrophil. When bacteria enter the body, neutrophils migrate to the site of infection or inflammation through the blood vessels, then through intertial tissue. Neutrophils are produced in the bone marrow and are fully mature when they are released into the blood stream.