 Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, constituting 50 % to 60 % of the total circulating white blood cells. One litre of human blood contains about five billion neutrophils, which are about 10 micrometers in diameter and live for only about five days. Once they have received the appropriate signals, it takes them about thirty minutes to leave the blood and reach the site of an infection. They are ferocious eaters and rapidly engulf invaders coated with antibodies and complement, and damaged cells or cellular debris. Neutrophils do not return to the blood; they turn into pus cells and die. Mature neutrophils are smaller than monocytes and have a segmented nucleus with several sections; each section is connected by chromatin filaments — neutrophils can have 2 – 5 segments. Neutrophils do not normally exit the bone marrow until maturity but during an infection neutrophil precursors called myelocytes and promyelocytes are released.
