Cellmediated Immunity

Frank W Fitch, The Ben May Institute, Division of Biological Sciences, The University of Chicago, Chicago, USA

Copyright © 1998 Elsevier Ltd. All Rights Reserved.

Cell-mediated immunity is mediated by T lymphocytes; humoral immunity, by antibodies. These distinctions are based on observations that delayed-type hypersensitivity (DTH) and other manifestations of cell-mediated immunity can be adoptively transferred by lymphoid cells but not by serum from sensitized animals, while other manifestations of immunity can be passively transferred by serum from immune animals. Cell-mediated and humoral immunity can exist in isolation: some chemicals induce contact sensitivity, a manifestation of cellmediated immunity, but do not induce production of antibodies, and some antigens can stimulate antibody production in the apparent absence of T lymphocytes. However, cell-mediated and humoral immunity usually develop concurrently in vivo, and the two responses often act synergistically. For example, although DTH can be adoptively transferred with T lymphocytes, the intensity of the reaction is usually greater if antibody is also passively administered. There also are conceptual difficulties inherent in these definitions: antibodies are produced by cells, and cells participate in many of the protective reactions mediated by antibodies. Also, T lymphocytes carry out many of their functions through soluble products which they secrete following stimulation with antigen.

In the early 1880s, Elie Metchnikoff proposed that inflammation had a protective role in responses to pathogens, and that phagocytic cells, rather than being harmful, constituted the primary line of defense through their ability to ingest and digest invading organisms. However, the contrary view, that humoral antibody played the dominant role, was furthered by the demonstration of bactericidal properties of immune serum and the protective effects of antibodies against bacterial exotoxins. Although in the early 1900s the phenomenon of DTH was shown to be unrelated to circulating antibody, it proved difficult to determine with certainty the relative contributions of humoral and cellmediated immunity until the central involvement of lymphocytes in immune responses was established finally in the late 1950s. In the early 1960s, several serendipitous observations led to the identification of two separate lymphocyte lineages, based on their development pathways: B lymphocytes, which originate in the bursa of Fabricius in birds and in the bone marrow of mammals; and T lymphocytes, which develop within the thymus from precursors originating initially in the yolk sac and later in the bone marrow.

These two developmentally distinct sets of lymphocytes were soon shown to have different functions: B cells display and secrete antibody molecules which react with intact antigen molecules; T cells react with processed antigen fragments which are associated with molecules of the major histocompatibility complex (MHC). T lymphocytes carry out some functions, such as the lysis of target cells that express the appropriate antigens, through direct cell-to-cell contact, but they exert many of their effector functions through secreted hormone-like proteins, called cytokines, which modulate the activities of various cells, including lymphocytes and phagocytes.

Although the classical manifestations of cellmediated immunity are phenomena which can be observed only in vivo, it is difficult to dissect the cellular and biochemical processes involved in these reactions in animals. Therefore, a variety of responses by antigen-stimulated T lymphocytes have been used as surrogate cell-mediated responses in vitro. It is generally assumed that these responses -which include T lymphocyte proliferation, cytolytic activity, 'helper' functions, and/or secretion of cytokines - relate to manifestations of cell-mediated immunity observed in vivo. However, none of the phenomena of cell-mediated immunity observed in sensitized animals have been characterized fully in cellular and biochemical terms.

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