TYPES OF T CELLS
Each of these cells has important functions in an immune response. The helper T cell, like other immune cells, secretes signals called cytokines that trigger the proliferation of both T cells and B cells. Helper T cells also secrete cytokines that enhance the immune response. Memory T cells are maintained in the body, and when the antigen is encountered again, these cells quickly begin to respond, giving rise to additional helper, memory, and cytotoxic T cells. The cytotoxic T cells are the true effector cells in cell-mediated immunity. These cells secrete granules containing chemicals that destroy a targeted cell.

Because all nucleated cells possess MHC class I molecules on their surfaces, cell-mediated immunity is a powerful means of detecting virally infected cells. Viral proteins associate with MHC class I molecules; when this complex is displayed on the surface of a cell, activated cytotoxic T cells are able to bind and kill such cells. Interestingly, the surface of cancer cells display altered proteins that, when joined with MHC class I molecules, trigger a cell-mediated response. Thus, by harvesting T cells that recognize antigens from the cancer cell and growing large numbers of these cells in the laboratory for reintroduction into the body, it is possible to use the immune system to fight cancers.