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PART 4 Regulation and Maintenance

in both allergic and inflammatory reactions. Basophils containlarge amounts of histamine (see chapter 22), which they releasewithin tissues to increase inflammation. They also release heparin, which inhibits blood clotting.

ASSESS YOUR PROGRESS

22. What are the two major functions of white bloodcells? Define ameboid movement, diapedesis, and chemotaxis.

23. Describe the morphology of the five types of white bloodcells.

24. Name the two white blood cells that function primarily asphagocytic cells. What are lysozymes?

25. Which white blood cell defends against parasitic worms?

26. Which white blood cell releases histamine and promotesinflammation?

27. B cells and T cells are examples of which type of whiteblood cell? How do these cells protect against bacteriaand viruses?

Lymphocytes

Lymphocytes comprise 20–25% of white blood cells (table 19.2).They are the smallest white blood cells, usually slightly larger indiameter than red blood cells. A lymphocyte’s cytoplasm consistsof only a thin, sometimes imperceptible, ring around the nucleus.Although lymphocytes originate in red bone marrow, they migratethrough the blood to lymphatic tissues, where they can proliferateand produce more lymphocytes. The majority of the body’s totallymphocyte population is in the lymphatic tissues: the lymphnodes, spleen, tonsils, lymphatic nodules, and thymus.Some specific types of lymphocytes play important roles inimmunity (see chapter 22). For example, B cells are a type oflymphocyte that can be stimulated by bacteria or toxins to divideand form cells that produce antibodies, a class of plasma proteinsalso called immunoglobulins. Antibodies can attach to bacteriaand activate mechanisms that destroy the bacteria. T cells areanother type of lymphocyte that protect against viruses and otherintracellular microorganisms by attacking and destroying the cellsin which they are found. In addition, T cells are involved in thedestruction of tumor cells and in tissue graft rejections.

Platelets

Platelets , or thrombocytes (table 19.2; see figure 19.7), are minutefragments of cells. They consist of a small amount of cytoplasmsurrounded by a plasma membrane. Platelets are roughly disc-shaped and average about 3 μm in diameter. Glycoproteins andproteins on their surface allow platelets to attach to other mole-cules, such as collagen in connective tissue. Some of these surfacemolecules, as well as molecules released from granules in theplatelet cytoplasm, play important roles in controlling blood loss.The platelet cytoplasm also contains actin and myosin, which cancause contraction of the platelet (see “Clot Retraction and Disso-lution” in section 19.5).The life expectancy of platelets is about 5–9 days. Plateletsare derived from megakaryocytes (meg- ă -kar′ ē - ō -sitz), whichare extremely large cells found in the red bone marrow. Smallfragments of these cells break off and enter the blood asplatelets.Platelets play an important role in preventing blood loss by(1) forming platelet plugs that seal holes in small vessels and(2) promoting the formation and contraction of clots that help sealoff larger wounds in the vessels.

ASSESS YOUR PROGRESS

28. What is a platelet? How do platelets form?

29. What are the two major roles of platelets in preventingblood loss?

Monocytes

Monocytes comprise 3–8% of white blood cells (table 19.2). Theyare typically the largest of the white blood cells. Monocytes nor-mally remain in the blood for about 3 days. Then they leave theblood and are transformed into macrophages. Macrophages migratethrough various tissues, where they phagocytize bacteria, deadcells, cell fragments, and other debris. An increase in the number ofmonocytes in the blood is often associated with chronic infection.Macrophages also stimulate responses from other cells in twoways: (1) by releasing chemical messengers and (2) by phagocytiz-ing and processing foreign substances, which are then presented tolymphocytes. The responses of these other cells help protect againstmicroorganisms and other foreign substances (see chapter 22).

Predict 5

Based on their morphology, identify each of the white blood cells shownin figure 19.8.

(a)

(b)

(c)

(d)

(e)

FIGURE 19.8 Identification of White Blood Cells

See Predict question 5.