Agranular leukocytes have globular, nonlobed nuclei and lack prominent cytoplasmic granules when viewed with the light micro scope. The two general types are monocytes and lymphocytes.
Although lymphocytes are the corner stone of the third line of defense, which is the subject of chapter 15, their origin and morphology are described here so their relationship to the other blood components is clear. Lymphocytes are the second most common WBC in the blood, comprising 20% to 35% of the total circulating leukocytes. The fact that their overall number throughout the body is among the highest of all cells indicates how important they are to immunity. One estimate suggests that about one-tenth of all adult body cells are lymphocytes, exceeded only by erythrocytes and fibroblasts.
In a stained blood smear, most lymphocytes appear as small, spherical cells with a uniformly dark, rounded nucleus surrounded by a thin fringe of clear cytoplasm, although in tissues they can become much larger and can even mimic monocytes in appearance. Two types of lymphocytes exist, B lymphocytes (B cells, for short) and T lymphocytes (T cells, for short). B cells were named for the bursa of Fabricius, the organ in birds in which they develop. In humans, who have no bursa, B cells are produced in the bone marrow. T cells are also produced in the bone marrow but mature in the thymus gland. Both populations of cells are transported by the bloodstream and lymph and move about freely between lymphoid organs and connective tissue. Natural killer (NK) cells develop from the same pre cursor as T and B cells but follow a different developmental path.
B and T lymphocytes are the key cells of the third line of defense, the specific immune response. When stimulated by foreign substances (antigens), lymphocytes are transformed into activated cells that neutralize and destroy those foreign substances. Activated B cells form specialized plasma cells, which in turn produce antibodies. Antibodies are proteins that bind to foreign cells or molecules and participate in their destruction. Because of these actions, B cells are said to contribute to antibody-mediated immunity (also referred to as humoral immunity). Activated T cells engage in a spectrum of immune functions—called cell mediated immunity—which include killing foreign cells directly and exerting control over immune function. Both classes of lymphocytes display specificity and memory, key characteristics of the immune system. Unlike B and T cells, natural killer cells never develop specificity for a single antigen and are generally not considered part of the third line of defense. The specific immune response modulated by lymphocytes is so important that most of chapter 15 is devoted to their reactions.
Monocytes are generally the largest of all white blood cells and the third most common in the circulation (3% to 7%). As a monocyte matures, its nucleus becomes oval- or kidney-shaped—indented on one side, off center, and often contorted with fine wrinkles. The cytoplasm holds many fine vacuoles containing digestive enzymes. Monocytes are discharged by the bone marrow into the bloodstream, where they live as phagocytes for a few days. Later they leave the circulation to undergo final differentiation into macrophages. Macrophages are key players in immunity, in general, responsible for
1. many types of specific and nonspecific phagocytic and killing functions (they assume the job of cellular housekeepers, “mop ping up the messes” created by infection and inflammation);
2. processing foreign molecules and presenting them to lymphocytes; and
3. secreting biologically active compounds that assist, mediate, attract, and inhibit immune cells and reactions.
We touch upon these functions in several ensuing sections.
Another product of the monocyte cell line is dendritic cells, named for their long, thin cell processes. Immature dendritic cells move from the blood to the mononuclear phagocyte system and lymphatic tissues, where they trap pathogens. Ingestion of bacteria and viruses stimulates dendritic cells to migrate to lymph nodes and the spleen to participate in reactions with lymphocytes.
Some of the major characteristics of leukocytes are summarized in table 1.
Table1. Characteristics of Leukocytes
Erythrocyte and Platelet Lines These elements stay in the circulatory system proper and are not generally considered part of the immune system. Their development is also shown in figure 1.
Fig1. Simplified diagram of blood cell and platelet development. The details of several intermediate steps have been omitted. Undifferentiated stem cells in the red marrow give rise to several different cell lines that become increasingly specialized until mature cells are released into circulation.
Erythrocytes are produced by stem cells that go through several stages of differentiation and lose their nucleus just before entering the circulation. The resultant red blood cells are simple, biconcave sacs of hemoglobin that transport oxygen and carbon dioxide to and from the tissues. These are the most numerous of circulating blood cells, appearing in stains as small pink circles. Red blood cells do not ordinarily have immune functions, though they can be the target of immune reactions.
Platelets are formed elements in circulating blood that are not whole cells. They are formed by the disintegration of a large, multinucleate cell, the megakaryocyte. In stains, platelets are blue-gray with fine red granules and are readily distinguished from cells by their small size. Platelets function primarily in hemostasis (plugging broken blood vessels to stop bleeding) and in releasing chemicals that act in blood clotting and inflammation.
