Jex

Lysosomes

(a) Chemoattractants, such as C5a, attract phagocyte to organisms to be ingested

(b) C3b coats organisms and attaches to C3b receptors on phagocyte

Lysosomes

(b) C3b coats organisms and attaches to C3b receptors on phagocyte

Phagolysosome

(g) Contents of phagolysosome eliminated by exocytosis

(c) Organism is engulfed into a phagosome

(d) Phagosome fuses with lysosome to produce phagolysosome

(e) Organism is killed within the phagolysosome

(f) Digestion and breakdown of organism within phagolysosome

(c) Organism is engulfed into a phagosome

(d) Phagosome fuses with lysosome to produce phagolysosome

(e) Organism is killed within the phagolysosome

(f) Digestion and breakdown of organism within phagolysosome

(g) Contents of phagolysosome eliminated by exocytosis

■ Engulfment. The phagocytic cell engulfs the invader, forming a membrane-bound vacuole called a phagosome. This process involves rearrangement of the phagocyte's cytoskeleton, forming armlike extensions called pseudopods that surround the material being engulfed. Engulfment itself does not destroy the microbe. ■ cytoskeleton, p. 73 ■ pseudopod, p. 73

■ Fusion of the phagosome with the lysosome. Within the phagocyte, the phagosome is transported along the cytoskeleton to a point where it can fuse with a lysosome, a membrane-bound body filled with various digestive enzymes, including lysozyme and proteases. The fusion results in the formation of a phagolysosome. In neutrophils, the membrane-bound bodies are referred to as granules.

■ Destruction and digestion. Within the fusion product, oxygen consumption increases enormously as sugars are oxidized via the TCA cycle, with the production of highly toxic oxygen products such as superoxide, hydrogen peroxide, singlet oxygen and hydroxyl radicals. As the available oxygen in the phagolysosome is consumed, the metabolic pathway switches to fermentation with the production of lactic acid, lowering the pH. Various enzymes degrade the peptidoglycan of the bacterial cell walls, and other components of the cell. ■ TCA cycle, pp. 137,144 ■ superoxide, p. 89 ■ hydrogen peroxide, p. 89

■ Exocytosis. Following the digestion of the microorganisms, the membrane-bound vesicle containing the digested material fuses with the plasma membrane. This expels the material to the external environment. ■ exocytosis, p. 72

Specialized Attributes of Macrophages

Macrophages can be viewed as the scavengers and sentries— routinely phagocytizing dead cells and debris, but always on the lookout, ready to destroy invaders, and able to call in reinforcements when needed. They are always present in tissues to at least some extent, where they either slowly wander or remain stationary. Macrophages that remain fixed in tissues are often referred to by different names according to the type of tissue in which they reside (see figure 15.5). These phagocytic cells play an essential role in every major tissue in the body.

Macrophages live for weeks to months, and maintain their killing power by continually regenerating their lysosomes. As macrophages die they are continually replaced by circulating monocytes that leave the blood and migrate to the tissues; recall that monocytes can differentiate into macrophages. Migration of monocytes is enhanced by certain stimuli associated with invasion and tissue damage.

Macrophages have several important characteristics that enable them to accomplish their diverse tasks. Various toll-like receptors enable them to sense material that signifies danger. When these receptors are triggered, the macrophage produces pro-inflammatory cytokines to alert and stimulate various other cells of the immune system. A macrophage can increase its otherwise limited killing power with the assistance of a subset of T-helper cells to become an activated macrophage. This is an example of the cooperation between the innate and adaptive

15.7 Inflammation—A Coordinated Response to Invasion or Damage 385

host defenses. Activation of macrophages induces the production of nitric oxide (NO) and oxygen radicals, which more effectively destroy microbes. These products also damage tissues when they are released, a reason why it would be detrimental for macrophages to continually maintain an activated state. Details of the activation process, including the roles of T-helper cells, will be discussed in chapter 16. ■ macrophage activation, p. 409

If activated macrophages fail to destroy microbes and chronic infection ensues, large numbers of macrophages can fuse together to form giant cells. Macrophages, giant cells, and T-helper cells form concentrated groups of cells called granulomas that wall off and retain organisms or other material that cannot be destroyed by the cells; again, this is an example of the cooperation between defense systems. This prevents the microbes from escaping to infect other cells (see figure 23.18). Granulomas are part of the disease process in tuberculosis, histoplasmosis, and other illnesses. ■ tuberculosis, p. 580 ■ histoplasmosis, p. 592

Specialized Attributes of Neutrophils

Neutrophils can be viewed as the rapid response team—quick to move into an area of trouble so that the offending invaders can be removed. They play a critical role during the early stages of inflammation, being the first cell type recruited from the bloodstream to the site of damage. They inherently have more killing power than macrophages, including those that have been activated. The cost for their effectiveness, however, is a relatively short life span of only 1 to 2 days in the tissues; once they have expended their granules, they die. Many more are in reserve, however, for it is estimated that for every neutrophil in the circulatory system, 100 more are waiting in the bone marrow, ready to be mobilized when needed.

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