Leukocytes (White Blood Cells)
The leukocytes, also called white blood cells, are the mobile units of the body’s protective system. They are formed partially in the bone marrow (granulocytes and monocytes and a few lymphocytes) and partially in the lymph tissue (lym-phocytes and plasma cells). After formation, they are transported in the bloodto different parts of the body where they are needed.
The real value of the white blood cells is that most of them are specifically transported to areas of serious infection and inflammation, thereby providing a rapid and potent defense against infectious agents. As we see later, the granu-locytes and monocytes have a special ability to “seek out and destroy” a foreign invader.
Types of White Blood Cells. Six types of white blood cells are normally present inthe blood. They are polymorphonuclear neutrophils, polymorphonucleareosinophils, polymorphonuclear basophils, monocytes, lymphocytes, and, occa-sionally, plasma cells. In addition, there are large numbers of platelets, which are fragments of another type of cell similar to the white blood cells found in the bone marrow, themegakaryocyte. The first three types of cells, the polymor-phonuclear cells, all have a granular appearance, as shown in cell numbers 7, 10, and 12 in Figure 33–1, for which reason they are called granulocytes, or, in clinical terminology, “polys,” because of the multiple nuclei.
The granulocytes and monocytes protect the body against invading organisms mainly by ingesting them—that is, by phagocytosis. The lymphocytes and plasma cells function mainly in connection with the immune system;. Finally, the function of platelets is specifically to activate the blood clotting mechanism.
Concentrations of the Different White Blood Cells in the Blood.
The adult human being has about 7000 white blood cells per microliter of blood (in comparison with 5 million red blood cells). Of the total white blood cells, the normal percentages of the different types are approximately the following:
The number of platelets, which are only cell frag-ments, in each microliter of blood is normally about 300,000.
Early differentiation of the pluripotential hematopoi-etic stem cell into the different types of committed stem cells is shown in Figure 32–2.
Aside from those cells committed to form red blood cells, two major lineages of white blood cells are formed, the myelocytic and the lymphocytic lineages. The left side of Figure 33–1 shows themyelocyticlineage, beginning with the myeloblast; the rightshows the lymphocytic lineage, beginning with the lymphoblast.
The granulocytes and monocytes are formed only in the bone marrow. Lymphocytes and plasma cells are produced mainly in the various lymphogenous tissues—especially the lymph glands, spleen, thymus, tonsils, and various pockets of lymphoid tissue elsewhere in the body, such as in the bone marrow and in so-called Peyer’s patches underneath the epithelium in the gut wall.
The white blood cells formed in the bone marrow are stored within the marrow until they are needed in the circulatory system. Then, when the need arises, various factors cause them to be released (these factors are discussed later). Normally, about three times as many white blood cells are stored in the marrow as circulate in the entire blood.This represents about a 6-day supply of these cells.
The lymphocytes are mostly stored in the various lymphoid tissues, except for a small number that are temporarily being transported in the blood.
As shown in Figure 33–1, megakaryocytes (cell 3) are also formed in the bone marrow. These megakary-ocytes fragment in the bone marrow; the small frag-ments, known as platelets (or thrombocytes), then pass into the blood. They are very important in the initia-tion of blood clotting.
The life of the granulocytes after being released from the bone marrow is normally 4 to 8 hours circulating in the blood and another 4 to 5 days in tissues where they are needed. In times of serious tissue infection, this total life span is often shortened to only a few hours because the granulocytes proceed even more rapidly to the infected area, perform their functions, and, in the process, are themselves destroyed.
The monocytes also have a short transit time, 10 to 20 hours in the blood, before wandering through the capillary membranes into the tissues. Once in the tissues, they swell to much larger sizes to becometissuemacrophages, and, in this form, can live for monthsunless destroyed while performing phagocytic func-tions. These tissue macrophages are the basis of the tissue macrophage system, discussed in greater detaillater, which provides continuing defense against infection.
Lymphocytes enter the circulatory system continu-ally, along with drainage of lymph from the lymph nodes and other lymphoid tissue. After a few hours, they pass out of the blood back into the tissues by dia-pedesis. Then, still later, they re-enter the lymph and return to the blood again and again; thus, there is con-tinual circulation of lymphocytes through the body. The lymphocytes have life spans of weeks or months; this life span depends on the body’s need for these cells.
The platelets in the blood are replaced about once every 10 days; in other words, about 30,000 platelets are formed each day for each microliter of blood.