Neutrophils are the most numerous type of white blood cell and they typically account for 50-70% of the total white blood cell population. Neutrophils are usually present in the blood but they can leave the circulation if they become chemically attracted to an extravascular site of damage or infection. They will typically ingest and kill five to twenty bacteria during their programmed life span of one to two days. A process called respiratory burst promotes the killing of bacteria. During this process, oxygen is actively metabolised to produce bacteria killing hydrogen peroxide, and defensin-mediated lysis occurs. Neutophils like other blood cells are formed in the bone marrow. Neutrophils are derived from a hemocytoblast, which differentiates in the bone marrow to form a myeloid stem cell, a myeloblast, a promyelocyte, a neutophilic promyelocyte and a neutrophilic band cell, which leaves the marrow and eventually differentiates in the circulation to become a neutrophil. It takes 6 to 9 days for a neutrophil to develop and they have a lifespan of 8 to 12 days.