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Cell-Mediated Immunity

CHAPTER CONTENTS

INTRODUCTION

Although humoral (antibody-mediated) immunity is an important host defense against many bacterial and viral diseases, in many other bacterial infections (especially intracellular infections such as tuberculosis) and viral infections, it is primarily the cell-mediated arm that imparts resistance and aids in recovery. Furthermore, cell-mediated immunity is important in defense against fungi, parasites, and cancers. It is also the main mechanism involved in the rejection of organ transplants. The strongest evidence for the importance of cell-mediated immunity comes from clinical situations in which its suppression (by immunosuppressive drugs or disease, e.g., acquired immunodeficiency syndrome [AIDS]) results in overwhelming infections or tumors.

The constituents of the cell-mediated immune system include several cell types: (1) macrophages, which present the antigen to T cells; (2) helper T cells, which participate in antigen recognition and in regulation (helper and suppressor) functions (see Chapter 58); (3) natural killer (NK) cells, which can inactivate pathogens; and (4) cytotoxic T cells, which can kill virus-infected cells with or without antibody. Macrophages and helper T cells produce cytokines that activate helper and cytotoxic T cells, leading to the killing of the pathogen or tumor cell.

Infection with some viruses, namely, measles virus and cytomegalovirus, can suppress cell-mediated immunity against other microorganisms. In particular, measles virus infection in people infected with Mycobacterium tuberculosis can result in a loss of purified protein derivative (PPD) skin test reactivity, reactivation of dormant organisms, and clinical disease. A proposed explanation for these findings is that when measles virus binds to its receptor on the surface of human macrophages, the production of interleukin-12 (IL-12) by the macrophages, which is necessary for cell-mediated immunity to occur, is suppressed.

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