The innate defense mechanisms distinguish between the kinds of carbohydrates that are produced by mammalian cells and those produced by bacteria. The bacterial carbohydrates that "flag" the cell for phagocytic attack are part of the glycoproteins and lipopolysaccharides on the bacterial cell wall.

There are three major groups of phagocytic cells: (1) neu-trophils; (2) the cells of the mononuclear phagocyte system, including monocytes in the blood and macrophages (derived from monocytes) in the connective tissues; and (3) organ-specific phagocytes in the liver, spleen, lymph nodes, lungs, and brain

Table 15.1

Structures and Defense Mechanisms of Nonspecific (Innate) Immunity





Physical barrier to penetration by pathogens; secretions contain lysozyme (enzyme that destroys bacteria)

Digestive tract

High acidity of stomach; protection by normal bacterial population of colon

Respiratory tract

Secretion of mucus; movement of mucus by cilia; alveolar macrophages

Genitourinary tract

Acidity of urine; vaginal lactic acid


Phagocytic cells

Ingest and destroy bacteria, cellular debris, denatured proteins, and toxins


Inhibit replication of viruses

Complement proteins

Promote destruction of bacteria; enhance inflammatory response

Endogenous pyrogen

Secreted by leukocytes and other cells; produces fever

The Immune System 447

(table 15.2). Organ-specific phagocytes, such as the microglia of the brain, are embryologically and functionally related to macrophages and may be considered part of the mononuclear phagocyte system.

The Kupffer cells in the liver, as well as phagocytic cells in the spleen and lymph nodes, are fixed phagocytes. This term refers to the fact that these cells are immobile ("fixed") in the walls of the sinusoids (chapter 13) within these organs. As blood flows through these wide capillaries of the liver and spleen, foreign chemicals and debris are removed by phagocytosis and chemically inactivated within the phagocytic cells. Invading pathogens are very effectively removed in this manner, so that blood is usually sterile after a few passes through the liver and spleen. Fixed phagocytes in lymph nodes similarly help to remove foreign particles from the lymph.

Connective tissues have a resident population of all leukocyte types. Neutrophils and monocytes in particular can be highly mobile within connective tissues as they scavenge for invaders and cellular debris. These leukocytes are recruited to the site of an infection by a process called chemotaxis—movement toward chemical attractants. The chemical attractants are a subclass of cytokines (autocrine/paracrine regulators—see chapter 11) known as chemokines. Neutrophils are the first to arrive at the site of an infection; monocytes arrive later and can be transformed into macrophages as the battle progresses.

If the infection has spread, new phagocytic cells from the blood may join those already in the connective tissue. These new neutrophils and monocytes are able to squeeze through the tiny gaps between adjacent endothelial cells in the capillary wall and enter the connective tissues. This process, called extravasation (or diapedesis) is illustrated in figure 15.1.

Phagocytic cells engulf particles in a manner similar to the way an amoeba eats. The particle becomes surrounded by cyto-plasmic extensions called pseudopods, which ultimately fuse. The particle thus becomes surrounded by a membrane derived from the plasma membrane (fig. 15.2) and contained within an organelle analogous to a food vacuole in an amoeba. This vacuole then fuses with lysosomes (organelles that contain digestive enzymes), so that the ingested particle and the digestive enzymes still are separated from the cytoplasm by a continuous membrane. Often, however, lysosomal enzymes are released before the food vacuole has completely formed. When this occurs, free lysosomal enzymes may be released into the infected area and contribute to inflammation.

Table 15.2 Phagocytic Cells and Their Locations

Phagocyte Location


Blood and all tissues



Tissue macrophages (histiocytes)

All tissues (including spleen, lymph

nodes, bone marrow)

Kupffer cells


Alveolar macrophages



Central nervous system

■ Figure 15.1 Stages involved in the migration of white blood cells from blood vessels into tissues. The figure depicts a neutrophil that goes through the stages of rolling, capture, adhesion and activation, and finally extravasation (diapedesis) through the blood vessel wall. This process is set in motion when the invading bacteria secrete certain chemicals, which attract and activate the white blood cells. The steps of extravasation require the binding of particular molecules on the white blood cell surface to receptor molecules on the surface of endothelial cells.

448 Chapter Fifteen



■ Figure 15.2 Phagocytosis by a neutrophil or macrophage. A phagocytic cell extends its pseudopods around the object to be engulfed (such as a bacterium). (Blue dots represent lysosomal enzymes.) (1) If the pseudopods fuse to form a complete food vacuole, lysosomal enzymes are restricted to the organelle formed by the lysosome and food vacuole. (2) If the lysosome fuses with the vacuole before fusion of the pseudopods is complete, lysosomal enzymes are released into the infected area of tissue.

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