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Cytotoxic T cells are a key part of the cellular immune response, killing cells that display foreign antigen on their surface, primarily virus-infected cells. Transformed cells can also be detected and eliminated by cytotoxic T cells. There are two mechanisms by which activated cytotoxic T cells kill cells presenting specific antigen. One method involves the release of secretory granules containing perforin and granzyme to induce lysis of the targeted cell. Cytotoxic T cells also express Fas ligand to bind to Fas on target cells and induce apoptosis. Communication with and interaction with other cell types is essential for cytotoxic T cell function. Distinct types of T cells are characterized and their activities determined by the proteins they express on their cell surface. Cytotoxic T cells will only respond to antigen presented on the surface of cells bound to MHC I proteins, not antigens present in solution. The T cell receptor, with the multiprotein CD3 complex, is responsible for the recognition of specific antigens, triggering the activation and proliferation of cells. Cytotoxic T cell activation also requires additional signals provided by helper T cells in addition to signals provided by antigen-presenting cells. Thy1 provides a general marker of T cells, and the presence of CD8 protein that binds to MHC distinguishes cytotoxic T cells from CD4 positive helper T cells. CD28 expressed by T cells acts as a costimulatory signal with the T cell receptor when it binds its ligand, a B-7 coreceptor, on antigen-presenting cells (see The Co-Stimulatory Signal During T-cell Activation). Inappropriate regulation of the costimulatory signal can lead to too great or too small of an immune response. Interaction of activated cytotoxic T cells is aided by increased expression of LFA-1, a cell-adhesion molecule that binds to ICAM on target cells. CD2 is a T cell adhesion molecule. Mice with a disrupted CD2 gene are largely normal and appear to have a normal immune system, but CD2 on cytotoxic T cells may assist in interaction with target cells. (This definition may be outdated - see the DesignNote.)
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