cell-mediated immunity(redirected from Type IV Hypersensitivity)
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cell-·me·di·at·ed im·mu·ni·ty (CMI),, cellular immunity
cell-me·di·at·ed im·mu·ni·ty, cellular immunity (CMI) (sel'mē'dē-āt-ĕd i-myū'ni-tē, sel'yū-lăr)
cell-mediated immunityAbbreviation: CMI.
Unlike B cells, T cells cannot recognize foreign antigens on their own. Foreign antigens are recognized by antigen-presenting cells (APCs) such as macrophages, which engulf them and display part of the antigens on the APC's surface next to a histocompatibility or “self-” antigen (macrophage processing). The presence of these two markers, plus the cytokine interleukin-1 (IL-1) secreted by the APCs activates CD4 helper T cells (TH cells), which regulate the activities of other cells involved in the immune response.
CMI includes direct lysis of target cells by cytotoxic T cells, creation of memory cells that trigger a rapid response when a foreign antigen is encountered for the second time, and delayed hypersensitivity to tissue and organ transplants. T cells also stimulate the activity of macrophages, B cells, and natural killer cells. These functions are controlled largely by the secretion of lymphokines such as the interleukins, interferons, and colony-stimulating factors. Lymphokines facilitate communication and proliferation of the cells in the immune system.