Page 827 - Fundamentals of anatomy physiology
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814 Unit 4 Fluids and Transport
proteins, also known as immunoglobulins. p. 683 B cells B cell spends about 30 hours in a lymph node, a small lymph-
are responsible for antibody-mediated immunity, which is also filtering structure, before moving on.
known as humoral (“liquid”) immunity because antibodies are
found in body fluids. Lymphocytes have relatively long life spans. About 80 per-
cent survive 4 years, and some last 20 years or more. Through-
Antibodies bind to specific chemical targets called an- out your life, you maintain normal lymphocyte populations
tigens, which stimulate the immune response. Most anti- by producing new lymphocytes in your red bone marrow and
gens are pathogens, parts or products of pathogens, or other lymphoid tissues.
foreign substances. Antigens are usually proteins, but some
lipids, polysaccharides, and nucleic acids are also antigens. Lymphocyte Production
The binding of an antibody to its target antigen starts a chain
reaction leading to the destruction of the target compound or In Chapter 19, we discussed hemopoiesis—the formation
organism. of the cellular elements of blood. pp. 682, 687, 698
In adults, red blood cell formation, or erythropoiesis, is
Natural killer cells are also known as large granular lym- normally confined to red bone marrow. In contrast, lym-
phocytes. NK cells provide innate (nonspecific) immunity. phocyte production, or lymphopoiesis (lim-fo. -poy-E. -sis),
They attack foreign cells, cells infected with viruses, and cancer involves the red bone marrow, thymus, and peripheral lym-
cells that appear in normal tissues. Their continuous “polic- phoid tissues (Figure 22–6).
ing” of peripheral tissues has been called immune surveillance
(Figure 22–5). Red bone marrow plays the primary role in maintaining
normal lymphocyte populations. Hemocytoblasts divide in the
22 Life Span and Circulation of Lymphocytes bone marrow of adults to generate the lymphoid stem cells that
produce all types of lymphocytes. The red bone marrow pro-
The various types of lymphocytes are not evenly distributed duces two distinct populations of lymphoid stem cells.
in the blood, bone marrow, spleen, thymus, and peripheral
lymphoid tissues. The ratio of B cells to T cells varies among One group of lymphoid stem cells remains in the red
tissues and organs. For example, B cells are seldom found in the bone marrow and the other group migrates to the thymus
thymus, but T cells outnumber B cells in blood by a ratio of 8:1. (Figure 22–6). Lymphoid stem cells in the red bone mar-
row divide to produce immature B cells and NK cells. B cell
The lymphocytes in these organs are visitors, not residents. development involves intimate contact with large stromal
All types of lymphocytes move throughout the body. They wan- (stroma, a bed) cells in the red bone marrow. The cytoplasmic
der through tissues and then enter blood vessels or lymphatic extensions of stromal cells contact or even wrap around the
vessels for transport. developing B cells. Stromal cells produce an immune system
hormone, or cytokine, called interleukin-7. It promotes the dif-
T cells move quickly. For example, a wandering T cell may ferentiation of B cells. (We consider cytokines and their varied
spend about 30 minutes in the blood, 5–6 hours in the spleen, effects in a later section.)
and 15–20 hours in a lymph node. B cells, which are respon-
sible for antibody production, move more slowly. A typical As they mature, B cells and NK cells enter the bloodstream
and migrate to peripheral tissues (Figure 22–6). Most of the
Clinical Note B cells move into lymph nodes, the spleen, or other lymphoid
tissues. The NK cells patrol the body, moving through periph-
Cancer and the Lymphatic System Metastasizing can- eral tissues in search of abnormal cells.
cer cells commonly spread within lymphatic vessels. These
vessels occur in almost all portions of the body except the The second group of lymphoid stem cells migrates to the
central nervous system, and lymphatic capillaries offer thymus (Figure 22–6). These stem cells and their descendants
little resistance to the passage of cancer cells. As a result, develop and mature in an environment that is isolated from
the lymph nodes serve as “way stations” for migrating the general circulation by the blood–thymus barrier. Under
c ancer cells. For this reason, an analysis of lymph nodes the influence of thymic hormones, the lymphoid stem cells
can provide information on the spread of cancer cells. Such d ivide repeatedly, producing the various kinds of T cells. These
information helps determine the appropriate therapies. T cells undergo a selection process to ensure that they will not
Lymphomas are one group of cancers originating in the react to the body’s own healthy cells and cellular products.
cells of the lymphatic system. During this process up to 98 percent of developing T cells are
deselected and undergo apoptosis. When the development of
the surviving T cells nears completion, they re-enter the blood-
stream and travel to peripheral tissues, including lymphoid
tissues and organs, such as the spleen (Figure 22–6).
