Free macrophages and microphages function in similar ways PDF

Preview

Summary

Free macrophages and microphages function in similar ways...

Description

Free macrophages and microphages function in similar ways: ■ Both can move through capillary walls by squeezing between adjacent endothelial cells. This process is known as emigration. p. 671 The endothelial cells in an injured area develop membrane “markers” that signal passing blood cells that something is wrong. The phagocytes then attach to the endothelial lining and migrate into the surrounding tissues. ■ Both may be attracted to or repelled by chemicals in the surrounding fluids, a phenomenon called chemotaxis. They are particularly sensitive to chemicals known as cytokines that are released by other body cells and to chemicals released by pathogens. ■ For both, phagocytosis begins with adhesion, the attachment of the phagocyte to its target. In adhesion, receptors on the plasma membrane of the phagocyte bind to the surface of the target. Adhesion is followed by the formation of a vesicle containing the bound target (look back at Figure 3–22, p. 101). The contents of the vesicle are digested once the vesicle fuses with lysosomes or peroxisomes. All phagocytes function in much the same way, although their targets may differ. The life span of a phagocyte can be rather brief. For example, most neutrophils die before they have engulfed more than 25 bacteria, and during an infection a neutrophil may attack that many in an hour. Immune Surveillance The immune system generally ignores the body’s own cells unless they become abnormal in some way. Natural killer (NK) cells, also known as large granular lymphocytes, are responsible for recognizing and destroying abnormal cells when they appear in peripheral tissues. Their continuous “policing” of peripheral tissues has been called immune surveillance. The plasma membrane of an abnormal cell generally contains antigens that are not found on the membranes of normal cells. NK cells recognize an abnormal cell by detecting those antigens. NK cells are much less selective about their targets than are other lymphocytes: They respond to a variety of abnormal antigens that may appear anywhere on a plasma membrane. They also attack any membrane containing abnormal antigens. As a result, NK cells are highly versatile defenders. A single NK cell can attack bacteria in the interstitial fluid, body cells infected with viruses, or cancer cells. NK cells respond much more rapidly than T cells or B cells. The activation of T cells and B cells involves a complex and time-consuming sequence of events. NK cells respond immediately on contact with an abnormal cell. Activated NK cells react in a predictable way (Figure 22–11): 1 Recognition and Adhesion. If a cell has unusual components in its plasma membrane, an NK cell recognizes that cell as abnormal. This recognition activates the NK cell, which then adheres to its target cell. 2 Realignment of Golgi Apparatus. The Golgi apparatus moves around the nucleus until the maturing (trans) face points directly toward the abnormal cell. The process might be compared to the rotation of a tank turret to point the cannon toward the...