2.1.2 T-cells
  • T cell A lymphoid cell from the bone marrow that migrates to the thymus gland, where it develops into a mature differentiated lymphocyte that circulates between blood and lymph, serving as one of the primary cells of the immune response.
  • Immature T cells are called thymocytes. Mature T cells are “antigen specific”: T cell receptor (TCR) proteins on the surface of each T cell detect only one antigen. T cells are identified by surface protein markers called clusters of differentiation (CDs). All T cells have the CD3 marker; additional markers differentiate T subsets. CD4 T helper cells serve primarily as regulators, secreting cytokines that stimulate the activities of other white blood cells. CD8 T cells (cytotoxic T cells), effector cells that directly lyse (kill) organisms, are an important defense against viruses. Most CD8 T cells also produce gamma interferon (INFg), one of the strongest stimulators of macrophage activity. Natural killer cells, originally believed to be a subset of T cells, are now recognized as being a third type of lymphocyte.
  • T cells cannot recognize foreign antigens without the help of macrophages and other antigen- presenting cells (APCs), which change antigenic proteins into peptides that bind with major histocompatibility complex molecules. However, once the macrophage has helped them identify an antigen as “nonself,” T cells dominate the specific immune response, directing macrophages, B cells, and other T cells in the body's defense. T cells also play a major role in graft rejection and other type IV hypersensitivity reactions, as well as in tumor cell recognition and destruction, because of the unique antigens these cells carry.
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