T Cell Receptor Proteins

The antigens recognized by B lymphocytes may be either proteins or carbohydrates, but only protein antigens are recognized by most T lymphocytes. Unlike B cells, T cells do not make antibodies and thus do not have antibodies on their surfaces to serve as receptors for these antigens. The T cells do, however, have a different type of antigen receptor on their membrane surfaces, and these T cell receptors have been identified as molecules closely related to the immunoglobulins. The T cell receptors differ from the antibody receptors on B cells in a very important respect: the T cell receptors cannot bind to free antigens. In order for T lymphocytes to respond to foreign antigens, the antigens must be presented to the T cells on the membrane of antigen-presenting cells.

The chief antigen-presenting cells are macrophages and stellate-shaped dendritic cells (fig. 15.15) Dendritic cells arise from stem cells in the bone marrow, but migrate through the blood and lymph to almost every tissue. They are especially concentrated at potential sites where antigen-bearing microorganisms might enter, such as the skin, intestinal mucosa, and lungs. For example, the basal layer of the epidermis contains Langerhans cells, which are immature dendritic cells. These cells engulf protein antigens by pinocytosis, partially digest these proteins into shorter polypeptides, and then move these polypep-tides to the cell surface. At the cell surface, the foreign polypeptides are associated with molecules called histocompatability antigens (discussed in the next section). This allows the antigen-presenting cells to activate the T lymphocytes (fig. 15.15)

In order to interact with the correct T lymphocytes (those that have specificity for the antigen), however, the dendritic cells must migrate through lymphatic vessels to the secondary lymphoid organs, where they secrete chemokines to attract T lymphocytes. This migration affords the antigen-presenting cells the opportunity for a close encounter with the correct T lymphocytes.

In response to endotoxin, a molecule released by bacteria, and to cytokines such as interleukin-1 and gamma interferon, production of the enzyme nitric oxide synthase is induced within macrophages. As discussed in chapter 14, this enzyme catalyzes the formation of nitric oxide, which in excessive amounts may produce the hypotension of septic shock. A normal amount of nitric oxide, however, is required for macrophages to destroy bacteria and tumor cells.

460 Chapter Fifteen

Dendritic cell

Venule

460 Chapter Fifteen

Dendritic cell

Activation Cell Dendritic Cell

Venule vessel

Activated T cell

■ Figure 15.15 Migration of antigen-presenting dendritic cells to secondary lymphoid organs activates T cells. Once the T cells have been activated by antigens presented to them by the dendritic cells, the activated cells divide to produce a clone. Some of these cells then migrate from the lymphoid organ into the blood. Once in the blood, these activated T cells can home in on the site of the infection because of chemoattractant molecules produced during the inflammation.

Activated T cell

■ Figure 15.15 Migration of antigen-presenting dendritic cells to secondary lymphoid organs activates T cells. Once the T cells have been activated by antigens presented to them by the dendritic cells, the activated cells divide to produce a clone. Some of these cells then migrate from the lymphoid organ into the blood. Once in the blood, these activated T cells can home in on the site of the infection because of chemoattractant molecules produced during the inflammation.

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • yusef
    What is the activation order of tcells?
    5 years ago
  • prisca
    What protiens make up the t cell receptors that interact with antigens?
    5 years ago

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