Neutrophils are Leukocytes (white blood cells) of the Polymorphonuclear Leukocyte subgroup.

Neutrophils form a primary defense against bacterial infection. Like all the cells of the immune system , neutrophils are produced in the bone marrow and circulate in the bloodstream. However, neutrophils move out of blood vessels into infected tissue in order to attack the foreign substance ( allergen , bacteria, etc.). Normally a serious bacterial infection causes the body to produce an increased number of neutrophils, resulting in a higher than normal WBC count . Neutrophils perform their function partially through phagocytosis, a process by which they "eat" other cells and foreign substances. The pus in a boil (abscess) is made up mostly of neutrophils.