The Cells of the Immune System

The cells of the immune system can move from one part of the body to another, traveling through the body's circulatory systems like vehicles on an extensive interstate highway system. They are always found in normal blood, but their numbers usually increase during infections, recruited from reserves of immature cells that develop in the bone marrow. Some cells play dual functions, having crucial roles in both innate and adaptive immunity.

The formation and development of blood cells is called hematopoiesis (Greek for "blood" and "to make"). All blood cells, including those important in the body's defenses, originate from the same type of cell, the hematopoietic stem cell, found in the bone marrow (figure 15.4). Stem cells are induced to develop into the various types of blood cells by a group of cytokines called colony-stimulating factors. Some of the cells of the immune system are already mature as they circulate in the bloodstream, but others differentiate, developing functional properties, after they leave the blood and enter the tissues.

The general categories of blood cells and their derivatives include red blood cells, platelets, and white blood cells. Red blood cells, or erythrocytes, carry oxygen in the blood. Platelets, which are actually fragments arising from large cells called megakaryocytes, are important for blood clotting. White blood cells, or leukocytes, are important in all immune defenses. Leukocytes can be divided into three broad groups— granulocytes, mononuclear phagocytes, and lymphocytes (table 15.2).

Granulocytes

Granulocytes all contain prominent cytoplasmic granules, filled with biologically active chemicals that are important in their function. There are three types of granulocytes—neutrophils, basophils, and eosinophils; their names reflect the staining properties of their cytoplasmic granules. Characteristics of granulocytes are described here:

■ Neutrophils are the most abundant and important granulocytes of the innate responses and are by far the best understood; they are sometimes called polymorphonuclear neutrophilic leukocytes, polys, or PMNs, names that reflect the appearance of multiple lobes of their single nucleus. They normally account for over 50% of circulating leukocytes, and their numbers increase during most acute bacterial infections. There are generally few in tissues except during inflammation and in reserve locations. Neutrophils are professional phagocytes; they are highly efficient at phagocytizing and destroying foreign material, particularly bacteria, and damaged cells. The contents of their granules, which stain poorly, include many antimicrobial substances and degradative enzymes essential for destruction of materials that the cell engulfs. Because of the importance of neutrophils in innate immunity, they will be described in more detail later in the chapter. ■ specialized attributes of neutrophils, p. 385

■ Basophils are blood cells involved in allergic reactions and inflammation. Their granules, which are stained dark purplish-blue by the basic dye methylene blue, contain histamine and other chemicals that increase capillary permeability during inflammation. Mast cells are similar in appearance and function to basophils but they are found in virtually all tissues, rather than in blood. They do not come from the same precursor cells as basophils. Mast cells are important in the inflammatory response and are responsible for many allergic reactions.

■ Eosinophils are thought to be primarily important in expelling parasitic worms from the body. They seem to be involved in allergic reactions, causing some of the symptoms associated with allergies, but reducing others. Relatively few eosinophils are found in blood, because most leave the bloodstream, ultimately entering local secretions. The granules of eosinophils, which are stained red by the acidic dye eosin, contain antimicrobial substances and also histaminase, an enzyme that breaks down histamine.

Nester-Anderson-Roberts: I III. Microorganisms and I 15. The Innate Immune I I © The McGraw-Hill

Microbiology, A Human Humans Response Companies, 2003

Perspective, Fourth Edition

Red blood cells (erythrocytes)

15.3 The Cells of the Immune System 377

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