T cells also express a number of other molecules that serve important functions in immune responses. CD4 and CD8 are expressed on distinct T-cell subsets and act as coreceptors during T-cell activation. During antigen recognition, CD4 molecules on T cells bind to invariant portions of class II MHC molecules (described later) on selected antigen-presenting cells (APCs); in an analogous fashion, CD8 binds to class I MHC molecules. CD4 is expressed on 50% to 60% of T cells, whereas CD8 is expressed on about 40% of T cells. The CD4-and CD8-expressing T cells— called CD4+ and CD8+ cells, respectively—perform different but overlapping functions. CD4+ T cells are called helper T cells because they secrete soluble molecules http://ebooksmedicine.net
Neutralization of microbe, B lymphocyte B + phagocytosis, complement activation
Activation of macrophages
Microbial antigen presented by antigen-presenting cells Activation (proliferation B T and differentiation) of B T T and B lymphocytes
Killing of infected cell (CTL) T
Regulatory Suppression of T lymphocyte T immune response
Killing of infected cell cell
Fig. 5.4 The principal classes of lymphocytes and their functions. B lymphocytes and T lymphocytes are cells of adaptive immunity, and natural killer (NK) cells are cells of innate immunity. Several more classes of lymphocytes have been identified, including NK-T cells and so-called “innate lymphoid cells” (ILCs); the functions of these cells are not established.
(cytokines) that help B cells to produce antibodies and also help macrophages to destroy phagocytosed microbes. The central role of CD4+ helper cells in immunity is highlighted by the severe compromise that results from destruction of these cells by human immunodeficiency virus (HIV) infection (described later). CD8+ T cells also can secrete cytokines, but their most important role is to directly kill virus-infected cells and tumor cells; hence, they are called cytotoxic T lymphocytes (CTLs). Other important invariant proteins on T cells include CD28, which functions as the receptor for molecules called costimulators that are induced on APCs by microbes, and various adhesion molecules that strengthen the bond between the T cells and APCs and control the migration of the T cells to different tissues.
T cells that function to suppress immune responses are called regulatory T lymphocytes. This cell type is described later, in the context of tolerance of self antigens.
While most T cells express TCRs composed of α and β chains, a minority of peripheral blood T cells and many T cells associated with mucosal surfaces (e.g., lung, gastrointestinal tract) express TCRs that are composed of γ and δ chains, which are similar but not identical to α and β chains. Such γδ T cells, which do not express CD4 or CD8, recognize nonprotein molecules (e.g., bacterial lipoglycans), but their functional roles are not well understood. Another small population of T cells expresses markers of both T cells and NK cells. These so-called NKT cells recognize microbial glycolipids and may play a role in defense against some infections. The antigen receptors of γδ T cells and NKT cells recognize antigens independently of MHC molecules, and are much less diverse than the receptors of the more abundant CD4+ and CD8+ T cells. We will not discuss these rare lymphocyte populations  further.