FIGURE 2-1 Morphology of neutrophils, mast cells, basophils, and eosinophils. A, The light micrograph of a Wright-Giemsa-stained blood neutrophil shows the multilobed nucleus, because of which these cells are also called polymorphonuclear leukocytes, and the faint cytoplasmic granules. B, The light micrograph of a Wright-Giemsa-stained section of skin shows a mast cell (arrow) adjacent to a small blood vessel, identifiable by the red blood cell in the lumen. The cytoplasmic granules in the mast cell, which are stained purple, are filled with histamine and other mediators that act on adjacent blood vessels to promote increased blood flow and delivery of plasma proteins and leukocytes into the tissue. (Courtesy of Dr. George Murphy, Department of Pathology, Brigham and Women's Hospital, Boston, Massachusetts.) C, The light micrograph of a Wright-Giemsa-stained blood basophil shows the characteristic blue-staining cytoplasmic granules. (Courtesy of Dr. Jonathan Hecht, Department of Pathology, Brigham and Women's Hospital, Boston, Massachusetts.) D, The light micrograph of a Wright-Giemsa-stained blood eosinophil shows the characteristic segmented nucleus and red staining of the cytoplasmic granules.

cytoplasm contains granules of two types. The majority, called specific granules, are filled with enzymes such as lysozyme, collagenase, and elastase. These granules do not stain strongly with either basic or acidic dyes (hema-toxylin and eosin, respectively), which distinguishes neutrophil granules from those of two other types of circulating granulocytes, called basophils and eosino-phils. The remainder of the granules of neutrophils, called azurophilic granules, are lysosomes containing enzymes and other microbicidal substances, including defensins and cathelicidins, which we will discuss in Chapter 4. Neutrophils are produced in the bone marrow and arise from a common lineage with mononuclear phagocytes. Production of neutrophils is stimulated by granulocyte colony-stimulating factor (G-CSF). An adult human produces more than 1 x 1011 neutrophils per day, each of which circulates in the blood for only about 6 hours. Neutrophils may migrate to sites of infection within a few hours after the entry of microbes. If a circulating neutrophil is not recruited into a site of inflammation within this period, it undergoes apoptosis and is usually phagocytosed by resident macrophages in the liver or spleen. After entering tissues, neutrophils function for a few hours and then die.

Mononuclear Phagocytes

The mononuclear phagocyte system consists of cells whose primary function is phagocytosis and that play central roles in innate and adaptive immunity. The cells of the mononuclear phagocyte system originate from a common precursor in the bone marrow, circulate in the blood, and mature and become activated in various tissues (Fig. 2-2). The cell type in this lineage that enters the peripheral blood from the marrow is incompletely differentiated and is called the monocyte. Monocytes are 10 to 15 |im in diameter, and they have bean-shaped nuclei and finely granular cytoplasm containing lyso-somes, phagocytic vacuoles, and cytoskeletal filaments (Fig. 2-3). Monocytes are heterogeneous and consist of at least two subsets, which are distinguishable by cell surface proteins and kinetics of migration into tissues. One population is called inflammatory because it is rapidly recruited from the blood into sites of tissue inflammation. The other type may be the source of tissue resident macrophages and some dendritic cells.

Once they enter tissues, these monocytes mature and become macrophages. Macrophages in different tissues have been given special names to designate specific locations. For instance, in the central nervous system, they are called microglial cells; when lining the vascular sinusoids of the liver, they are called Kupffer cells; in pulmonary airways, they are called alveolar macrophages; and multinucleate phagocytes in bone are called osteoclasts.

Macrophages perform several important functions in innate and adaptive immunity.

• A major function of macrophages in host defense is to ingest and kill microbes. The mechanisms of killing, which are discussed in Chapter 4, include the enzymatic generation of reactive oxygen and nitrogen species that are toxic to microbes, and proteolytic digestion.

Bone marrow


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