Cytoplasm Of T Cell

lymphocytes can only recognize an antigen when it is "presented" to them by cells called antigen-presenting cells (APCs). Cytotoxic T lymphocytes can only recognize antigen on other body cells, such as cells transformed by cancer or infected with a virus.

When a helper T lymphocyte recognizes an antigen bound to a MHC molecule, the TCR attaches to the antigen-MHC complex. The binding of the TCR to the antigen-MHC complex then triggers the helper T lymphocyte to release immune chemicals, or cytokines, immune substances (proteins) that are biologic modulators of immune responses. The specific cytokines produced by helper CD4' T lymphocytes are called interleukins. Interleukins stimulate other T cells, B cells, and NK cells to differentiate and proliferate.

When a cytotoxic T lymphocyte recognizes an antigen-MHC complex and the TCR attaches to it, the cytotoxic T lymphocyte also releases cytokines that stimulate the cell to proliferate. These new cytotoxic T cells then seek out and destroy the abnormal host cells.

The two classes of MHC molecules display peptides on the surface of cells

MHC molecules display small fragments of digested foreign proteins on the surface of cells. These proteins bind to MHC molecules inside the cell and are then transported to the cell surface. MHC I and MHC II molecules are products of a "supergene" located on chromosome 6 in humans, known as the major bistocompatability gene complex. The expression of this gene complex produces molecules that are specific not only to the individual cell that produces them, but also to the tissue type and degree of cellular differentiation.

MHC I is expressed on the surface of all nucleated cells and platelets. MHC I molecules act as a target to allow the elimination of abnormal host cells (e.g., virus-infected or transformed cancer cells). MHC I molecules perform this function by displaying on their surface all of the peptides that are actively synthesized by the cell. Therefore, all endogenous "self" peptides are displayed on the surface of every cell in the body, but viral or cancer-specific peptides are displayed only on the surface of infected or transformed cells (Fig. 13.4).

MHC II is limited in its distribution (see Fig. 13.4). It is expressed on the surface of all APCs and is critical in immune interactions. The MHC II molecules present partially digested, endocytosed foreign peptides to helper CD4+ T lymphocytes.

CD8+ T lymphocytes are MHC 1 restricted, and CD4 ' T lymphocytes are MHC II restricted

MHC molecules are recognized by helper CD4+ T lymphocytes or cytotoxic CD8+ lymphocytes, depending on antigen-binding region antigen-binding region

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