Phagocytosis removes foreign particles from the lymph as it moves from the interstitial spaces to the bloodstream. Phagocytes in the blood vessels and in the tissues of the spleen, liver, or bone marrow usually remove particles that reach the blood. Recall from chapter 14 (p. 557) that the most active phagocytic cells of the blood are neutrophils and monocytes. These cells can leave the bloodstream by squeezing between the cells of the blood vessel walls (diapedesis). Chemicals released from injured tissues attract these cells (chemotaxis). Neutrophils engulf and digest smaller particles; monocytes phagocytize larger ones.

Monocytes are relatively nonmotile phagocytes that occupy lymph nodes, the spleen, liver, and lungs. Monocytes give rise to macrophages (histiocytes), which become fixed in various tissues and attach to the inner walls of blood and lymphatic vessels. A macrophage can engulf up to 100 bacteria, compared to the twenty or so bacteria that a neutrophil can engulf. Monocytes, macrophages, and neutrophils constitute the mononuclear phagocytic system (reticuloendothelial system). Table 16.3 summarizes the types of nonspecific defenses.

H What is an infection?

Explain six nonspecific defense mechanisms.

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