T CELL DEVELOPMENT AND MATURATION T cells go through a process that helps make sure that the cell won’t attack normal cells and that it belongs to the cell. Mature T cells will express one of two different molecules on their surface: CD4 molecules indicate that the cell is a helper T cell (which “helps” the immune process), while CD8 molecules indicate that the cell is a cytotoxic T cell (which does actual cell and pathogen killing). In the thymus cortex, the immature T cells do not express either molecule. They are referred to as “double negatives.” These cells bind to the class I MHC molecules on the epithelium of the thymus in a process of positive selection. This makes sure the T cell belongs to the person’s body. Cells that cannot bind to the MHC I molecule are destroyed and phagocytized. The thymocytes or T cells, as they mature, will become “double positives” that will express both CD4 and CD8 on their surface. They move onto the medulla of the thymus, participating in “negative selection” This will involve the presence of a professional antigen presenting cell inside the thymus. The cell that can bind to the profession APC is destroyed because it means that it will fight off a self-cell, causing an autoimmune disease. This means that only two percent of the initial number of thymocytes leave the thymus as mature and functioning T cells. Ultimately, the cell that leaves the thymus gland is only going to be a single positive. The CD4 cell is a helper T cell that will bind to a class II MHC cell, while the CD8 cell is a cytotoxic T cell that will bind to a class I MHC. Mature T cells become activated by recognizing an antigen attached to the self’s MHC molecule, causing it to divide rapidly in a process called clonal expansion. This enhances the immune response. They have the ability to bind to a specific pathogenic antigen, which is called clonal selection. The cells that are made by an activated T cell are clones “identical” to the original cell. Some of the dividing cells will be “memory” T cells, which last a long time and retain the memory of the infection. Others will be “effector” T cells that will respond to the infection.
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