The principal cells of the immune system are lymphocytes, antigen-presenting cells, and effector cells. Lymphocytes are the cells that specifically recognize and respond to foreign antigens and are therefore the mediators of humoral and cellular immunity. There are distinct subpopulations of lymphocytes that differ in how they recognize antigens and in their functions (Fig. 1-5). B lymphocytes are the only cells capable of producing antibodies. They recognize extracellular (including cell surface) antigens and differentiate into antibody-secreting plasma cells, thus functioning as the mediators of humoral immunity. T lymphocytes, the cells of cell-mediated immunity, recognize the antigens of intracellular microbes and either help phagocytes to destroy these microbes or directly kill the infected cells. T cells do not produce antibody molecules. Their antigen receptors are membrane molecules distinct from but structurally related to antibodies (see Chapter 7). T lymphocytes have a restricted specificity for antigens; they recognize peptides derived from foreign proteins that are bound to host proteins called major histocompatibility complex (MHC) molecules, which are expressed on the surfaces of other cells. As a result, these T cells recognize and respond to cell surface-associated but not soluble antigens (see Chapter 6). T lymphocytes consist of functionally distinct populations, the best defined of which are helper T cells and cytotoxic (or cytolytic) T lymphocytes (CTLs). In response to antigenic stimulation, helper T cells secrete proteins called cytokines, which are responsible for many of the cellular responses of innate and adaptive immunity and thus function as the "messenger molecules" of the immune system. The cytokines secreted by helper T lymphocytes stimulate the proliferation and differentiation of the T cells themselves and activate other cells, including B cells, macrophages, and other leukocytes. CTLs kill cells that produce foreign antigens, such as cells infected by viruses and other intracellular microbes. Some T lymphocytes, which are called regulatory T cells, function mainly to inhibit immune responses. A third class of lymphocytes, natural killer (NK) cells, is involved in innate immunity against viruses and other intracellular microbes. A small population of T lymphocytes that express a cell surface protein found on NK cells are called NKT cells; their specificities and role in host defense are not well understood. We will return to a more detailed discussion of the properties of lymphocytes in Chapter 2 and in later chapters. Different classes of lymphocytes can be distinguished by the expression of surface proteins that are named CD molecules and numbered (see Chapter 2).
The initiation and development of adaptive immune responses require that antigens be captured and displayed to specific lymphocytes. The cells that serve this role are called antigen-presenting cells (APCs). The most specialized APCs are dendritic cells, which capture microbial antigens that enter from the external environment, transport these antigens to lymphoid organs, and present the antigens to naive T lymphocytes to initiate immune responses. Other cell types function as APCs at different stages of cell-mediated and humoral immune responses. We will describe the functions of APCs in Chapter 6.
The activation of lymphocytes by antigen leads to the generation of numerous mechanisms that function to eliminate the antigen. Antigen elimination often requires the participation of cells that are called effector cells because they mediate the final effect of the immune response, which is to get rid of the microbes. Activated T lymphocytes, mononuclear phagocytes, and other leukocytes function as effector cells in different immune responses.
Lymphocytes and APCs are concentrated in anatomically discrete lymphoid organs, where they interact with one another to initiate immune responses. Lymphocytes are also present in the blood; from the blood, they can recirculate through lymphoid tissues and home to peripheral tissue sites of antigen exposure to eliminate the antigen (see Chapter 3).
The cells of innate immunity interact with one another and with other host cells during the initiation and effector stages of innate and adaptive immune responses. Many of these interactions are mediated by secreted proteins called cytokines. We will describe the properties and functions of individual cytokines when we discuss immune responses in which these proteins play important roles. We summarize some of the general features and functional categories of cytokines below.
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