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 (lymphocytes and plasma cells). After formation, they are transported in the blood to different parts of the body where they are needed.
The real value of WBCs 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 granulocytes and monocytes have a special ability to “seek out and destroy” a foreign invader.
GENERAL CHARACTERISTICS OF LEUKOCYTES
Types of White Blood Cells. Six types of WBCs are normally present in the blood: polymorphonuclear neutrophils, polymorphonuclear eosinophils, polymorphonuclear basophils, monocytes, lymphocytes, and, occasionally, plasma cells. In addition, there are large numbers of platelets, which are fragments of another type of cell similar to the WBCs found in the bone marrow, the mega karyocyte. The first three types of cells, the polymorphonuclear cells, all have a granular appearance, as shown in cell numbers 7, 10, and 12 in Figure 1, and for this reason they are called granulocytes or, in clinical terminology, “polys,” because of the multiple nuclei.
Fig1. Genesis of white blood cells. The different cells of the myelocyte series are 1, myeloblast; 2, promyelocyte; 3, megakaryocyte; 4, neutrophil myelocyte; 5, young neutrophil metamyelocyte; 6, “band” neutrophil metamyelocyte; 7, polymorphonuclear neutrophil; 8, eosinophil myelocyte; 9, eosinophil metamyelocyte; 10, polymorphonuclear eosinophil; 11, basophil myelocyte; 12, polymorphonuclear basophil; 13-16, stages of monocyte formation.
The granulocytes and monocytes protect the body against invading organisms by ingesting them (i.e., by phagocytosis) or by releasing antimicrobial or inflammatory substances that have multiple effects that aid in destroying the offending organism. The lymphocytes and plasma cells function mainly in connection with the immune system, as is discussed in Chapter 35. Finally, the function of platelets is specifically to activate the blood clotting mechanism, which is discussed in Chapter 37.
Concentrations of the Different White Blood Cells in the Blood. The adult human being has about 7000 WBCs per microliter of blood (in comparison with 5 million red blood cells [RBCs] per microliter). Of the total WBCs, the normal percentages of the different types are approximately the following:
The number of platelets, which are only cell fragments, in each microliter of blood is normally about 300,000.
GENESIS OF WHITE BLOOD CELLS
Early differentiation of the pluripotential hematopoietic stem cell into the different types of committed stem cells is shown in Figure 2 in the previous chapter. Aside from the cells committed to form RBCs, two major lineages of WBCs are formed, the myelocytic and the lymphocytic lineages. The left side of Figure 1 shows the myelocytic lineage, beginning with the myeloblast; the right side shows the lymphocytic lineage, beginning with the lymphoblast.
Fig2. Movement of neutrophils by diapedesis through capillary pores and by chemotaxis toward an area of tissue damage.
T he 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 else where in the body, such as in the bone marrow and in so-called Peyer’s patches underneath the epithelium in the gut wall.
The WBCs 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 WBCs are stored in the marrow as circulate in the entire blood. This quantity 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 1, megakaryocytes (cell 3) are also formed in the bone marrow. These megakaryocytes fragment in the bone marrow; the small fragments, known as platelets (or thrombocytes), then pass into the blood. They are very important in the initiation of blood clotting.
LIFE SPAN OF WHITE BLOOD CELLS
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 become tissue macrophages, and, in this form, they can live for months unless destroyed while performing phagocytic functions. These tissue macrophages are the basis of the tissue macrophage system (discussed in greater detail later), which provides continuing defense against infection.
Lymphocytes enter the circulatory system continually, 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 diapedesis. Then they re-enter the lymph and return to the blood again and again; thus, there is continual circulation of lymphocytes through the body. The lymphocytes have life spans of weeks or months, depending 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.
