Historically, two major types of storage granules were definitively identified by the presence or absence of MPO. The MPO-positive azurophil (primary) granules are formed only during the promyelocyte stage and are reduced in number by mitosis. It has recently been shown that the peroxidase negative granules constitute a continuum from early appearing granules (myelocyte stage) that contain lactoferrin but no gelatinase to granules that contain both lactoferrin and gelatinase (metamyelocyte stage) to granules that contain gelatinase but no lactoferrin (band cell stage). The aforementioned granules that contain lactoferrin are referred to as specific (secondary) granules while the granules that contain gelatinase but not lactoferrin are referred to as gelatinase (tertiary) granules. Thus, the mature circulating neutrophil contains the three major granule types. When appropriately stimulated, these cells move from blood to tissues and, within seconds, the granules may release their contents in endocytic vacuoles or, by fusion with the plasma membrane, to the exterior of the cell.