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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Understand that white blood cells work in concert to combat infection and to trigger an inflammatory response.

  • Describe the basic steps in elimination of infectious microorganisms by phagocytosis.

  • Describe the role of chemotaxis in leukocyte function.

  • List the key components found within the granules of phagocytes as well as basophils and their primary functions.

  • List the reactive oxygen species produced during the respiratory burst.

  • Explain the basis for the physiological effects caused by defects in the NADPH oxidase system.

  • Explain the molecular basis of type 1 leukocyte adhesion deficiency.

  • Describe how neutrophils and eosinophils entrap parasites using NETs (neutrophil extracellular traps).

  • Describe the role of the helper T cells in the production of new antibodies.

  • Define the term cytokine and describe the key characteristics of interleukins, interferons, prostaglandins, and leukotrienes.

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BIOMEDICAL IMPORTANCE

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White blood cells, or leukocytes, serve as key sentries and potent defenders against invading pathogens. The most abundant type of white blood cell, called neutrophils, ingest and destroy invading bacteria and fungi, a process known as phagocytosis. Larger parasites are phagocytized by eosinophils. Circulating monocytes migrate from the bloodstream to diseased tissues, where they differentiate into phagocytic macrophages. Granulocytes such as basophils and mast cells release stored effectors that attract additional leukocytes to the site of infection and trigger an inflammatory response. B lymphocytes generate and release protective antibodies with the assistance of T lymphocytes. Other lymphocytes, such as cytotoxic T cells and natural killer cells, target virally infected and malignantly transformed host cells.

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Malignant neoplasms of blood-forming tissues, called leukemias, can lead to the uncontrolled production of one or more of the major classes of white blood cells. The hyperactivation of granulocytes that occurs as part of an allergic response can, in extreme cases, lead to anaphylaxis and death. Damage to or infection of the bone marrow can lead to leukopenia, a depression in the production of white blood cells. The resulting deficit in the levels of circulating leukocytes can leave the affected individual vulnerable to infection (immunocompromised). Leukopenia can result from physical injury, chemotherapy, ionizing radiation, infection by the Epstein-Barr virus (mononucleosis), an autoimmune response (Lupus), or the displacement of bone marrow cells by fibrous tissues (myelofibrosis).

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DEFENSE AGAINST INFECTION REQUIRES MULTIPLE CELL TYPES

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The white blood cells, or leukocytes, are key participants in the acute inflammatory response, a multicomponent process that defends the body against infectious organisms and ameliorates the impact of tissue infection or morbidity. The principal steps in an inflammatory response include (1) an increase of vascular permeability, (2) the entry of activated leukocytes into the tissues, (3) activation of platelets, and (4) spontaneous subsidence (resolution) if the invading microorganisms have been dealt with successfully. Basophils secrete hematologic effectors such as ...

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