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13.49: Cell-Mediated Response

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    Do cells really attack other cells?

    They sure do. Depicted here is a group of T cells attacking a cancer cell. When they can, the T cells search out and destroy “bad” cells.

    Cell-Mediated Immune Response

    In addition to the humoral response, the other type of immune response is the cell-mediated immune response, which involves mainly T cells. It leads to the destruction of cells that are infected with viruses. Some cancer cells are also destroyed in this way. There are several different types of T cells involved in a cell-mediated immune response, including helper, cytotoxic, and regulatory T cells.

    T Cell Activation

    All three types of T cells must be activated by an antigen before they can fight an infection or cancer. T cell activation is illustrated in Figure below. It begins when a B cell or nonspecific leukocyte engulfs a virus and displays its antigens. When the T cell encounters the matching antigen on a leukocyte, it becomes activated. What happens next depends on which type of T cell it is.

    f-d_9e9f1d1238a48be70b6ea245b2d32ccb45186c1d0a7e59f4664fc5bd+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.jpgT cell activation requires another leukocyte to engulf a virus and display its antigen.

    Helper T Cells

    Helper T cells are like the “managers” of the immune response. They secrete cytokines, which activate or control the activities of other lymphocytes. Most helper T cells die out once a pathogen has been cleared from the body, but a few remain as memory cells. These memory cells are ready to produce large numbers of antigen-specific helper T cells like themselves if they are exposed to the same antigen in the future.

    Cytotoxic T Cells

    Cytotoxic T cells destroy virus-infected cells and some cancer cells. Once activated, a cytotoxic T cell divides rapidly and produces an “army” of cells identical to itself. These cells travel throughout the body “searching” for more cells to destroy. Figure below shows how a cytotoxic T cell destroys a body cell infected with viruses. This T cell releases toxins that form pores in the membrane of the infected cell. This causes the cell to burst, destroying both the cell and the viruses inside it.

    f-d_00bb5061f55e69bf2567b7cdf7b77d9f31c23e454b863de0e13a66da+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.jpgA cytotoxic T cell releases toxins that destroy an infected body cell and the viruses it contains.

    After an infection has been brought under control, most cytotoxic T cells die off. However, a few remain as memory cells. If the same pathogen enters the body again, the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.

    Regulatory T Cells

    Regulatory T cells are responsible for ending the cell-mediated immune response after an infection has been curbed. They also suppress T cells that mistakenly react against self antigens. What might happen if these T cells were not suppressed?

    Summary

    • Activated T cells destroy certain cancer cells and cells infected by viruses.
    • Memory T cells remain in the body after the immune response and provide antigen-specific immunity to the virus.

    Review

    1. Describe one way that cytotoxic T cells destroy cells infected with viruses.
    2. What are regulatory T cells?

    Resources

    Image Reference Attributions
    f-d_961e59cb1f1ccae95cc91507f42027998ef7f1e03dc3fea42b80c327+IMAGE_TINY+IMAGE_TINY.jpg [Figure 1] Credit: Laura Guerin;User:Fvasconcellos/Wikimedia Commons
    Source: CK-12 Foundation ; commons.wikimedia.org/wiki/File:Antibody.svg
    License: Public Domain
    f-d_9e9f1d1238a48be70b6ea245b2d32ccb45186c1d0a7e59f4664fc5bd+IMAGE_THUMB_SMALL_TINY+IMAGE_THUMB_SMALL_TINY.jpg [Figure 2] Credit: Courtesy of the National Institutes of Health and User:DO11.10/Wikimedia Commons;User:Fvasconcellos/Wikimedia Commons
    Source: commons.wikimedia.org/wiki/File:Antigen_presentation.jpg ; commons.wikimedia.org/wiki/File:Antibody.svg
    License: Public Domain
    f-d_00bb5061f55e69bf2567b7cdf7b77d9f31c23e454b863de0e13a66da+IMAGE_THUMB_SMALL_TINY+IMAGE_THUMB_SMALL_TINY.jpg [Figure 3] Credit: Laura Guerin;User:Fvasconcellos/Wikimedia Commons
    Source: CK-12 Foundation ; commons.wikimedia.org/wiki/File:Antibody.svg
    License: CC BY-NC 3.0; Public Domain
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