White cells are a remarkably versatile group of cells whose primary purpose is to defend against bacteria, viruses, fungi, or other foreign substances. To this end, most white cells are granulated and these granules contain enzymes used for digestion and destruction of the invading organisms. In the bone marrow there is a 4:1 ratio, the M:E ratio indicating that four myeloid, or white cells are produced for one erythroid cell. Daily production of white cells is 1.5 billion. Transit from the bone marrow to the peripheral circulation takes place only after white cells have been held in the maturation-storage pool of the bone marrow. For example, segmented neutrophils, the most mature of all of the white cells, are held for 7 to 10 days before their release into the peripheral circulation. Other white cell types have much shorter storage in the maturation pool time.1 Once released into the circulation, most white cells are short lived before they migrate into tissues. The white cells that are observed in the peripheral circulation are only a snapshot of white cells that are located in three distinct cell compartments: the bone marrow, the circulation, and the tissues.

White blood cells (WBCs) are referred to as leukocytes. For clarity, the word leukocytic applies to the white cells of all stages; granulocytic applies only to granulated white cells; and myelocytic is used in describing a particular white cell condition. The term myelocytic may also be used interchangeably for granulocytic in conditions such as chronic granulocytic leukemia or chronic mye-locytic leukemia. Suffice it to say that these three words—granulocytic, leukocytic, and myelocytic—are all used in denoting some stage of the white cell family. They are not meant to be confusing, but often are, despite good intentions.

WBCs, or leukocytes, have a more complex maturation cycle than erythrocytes. To begin, there is only one mature red cell form as opposed to five mature white cell forms. Red blood cells journey through the circulation for 120 days while white cells spend only hours in the circulating blood. Like red cells, white cells originate from the pluripotent stem cell. The pluripo-tent stem cell gives rise to the myeloid stem cell and the lymphoid stem cell. Through a series of interventions from interleukins (chemical stimulators) and growth factors, a CFU-GEMM is structured to give rise to gran-ulocytes, erythrocytes, monocytes, and macrophages. Curiously, megakaryocytes, eosinophils, and basophils have their own CFU: CGU-Meg and CFU-eosinophil/ basophil. Lymphocytes originate not only from the bone marrow but also from the thymus and thus, they have a distinctive place on the hematopoietic maturation chart (see Fig. 2.3).

Most of the function of the white cells is performed in the tissues, and it is here that white cells reside for 2 to 5 days. WBCs that appear in the circulation are part of two distinctive cell pools: the marginating pool and the circulating pool. The marginating pool designates those white cells that are located along the vessel endothelium ready to migrate to a site of injury or infection. The circulating pool designates those white cells actually in the bloodstream.2 At any particular point in the peripheral circulation, the white cells are evenly divided in either pool and there is rapid transfer from pool to pool. An additional site of white cell storage is the spleen, which harbors one fourth of the white cell population.

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