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After studying this chapter, you should be able to:

  • Understand the significance of immunity, particularly with respect to defending the body against microbial invaders.

  • Delineate the role and mechanisms of innate immunity.

  • Identify the roles of the humoral and cellular arms of acquired immunity, the cellular mediators of these responses, and the molecular basis for the recognition of a diverse set of antigens.

  • Understand the general principles governing regulation of immunity by soluble cytokines, chemokines and hematopoietic growth factors, as well as the complement system.

  • Define the roles of additional circulating and tissue cell types that contribute to immune and inflammatory responses, including granulocytes, mast cells, monocytes, and platelets. Describe how phagocytes are able to kill internalized bacteria.

  • Understand the basis of inflammatory responses and wound healing.


As an open system, the body is continuously called upon to defend itself from potentially harmful invaders such as bacteria, viruses, and other microbes. This is accomplished by the immune system, which is subdivided into innate and adaptive (or acquired) branches. The immune system is composed of specialized effector cells that sense and respond to foreign antigens and other molecular patterns not found in human tissues, as well as various regulatory molecules, including the large collection of soluble mediators known as cytokines. Likewise, the immune system clears the body’s own cells that have become senescent or abnormal, such as cancer cells. Finally, normal host tissues occasionally become the subject of inappropriate immune attack, such as in autoimmune diseases or in settings where normal cells are harmed as innocent bystanders when the immune system mounts an inflammatory response to an invader. It is beyond the scope of this volume to provide a full treatment of all aspects of modern immunology. Nevertheless, the student of physiology should have a working knowledge of immune functions and their regulation, due to a growing appreciation for the ways in which the immune system can contribute to normal physiologic regulation in a variety of tissues, as well as contributions of immune effectors to pathophysiology.



All multicellular organisms, including invertebrates and plants, express the ancient protective mechanism of innate immunity. This system is triggered by receptors that bind sequences of sugars, lipids, amino acids, or nucleic acids that are common in bacteria and other microorganisms, but are not found in eukaryotic cells. These receptors, in turn, activate various defense mechanisms. The receptors are coded in the germ line, and their fundamental structure is not modified by exposure to antigen. The activated defenses include, in various species, release of cytokines known as interferons, phagocytosis, production of antibacterial peptides, activation of the complement system, and several proteolytic cascades. This primitive immune system is important in vertebrates, particularly in the early response to infection. However, innate immunity is also complemented in vertebrates by adaptive or acquired immunity, a system ...

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