Cellular immunitv and humoral immunity have several similar features . T cells have membrane proteins that bind only to a specific antigen, just as B cells do. T cells rely on displaying fragments of antigens combined with their MHC pro-teins. T cells must be co-stimulated by interacting with other cells that present the same antigen-MHC combination. An antigenic challenge causes the co-stimulated cells to release cytokines that stimulate single T cells to divide to form clones of helper T cells and cytotoxic T cells.
Cytotoxic T cells are the effector cells of cellular immunity. They can cause the death of antigen-bearing cells in several ways, but all require that they have close contact. This explains why T lymphocytes circulate through the blood, connective tissues and lymph - they are searching out foreign or abnormal cells. This immune surveillance is aimed at cells infected by viruses, bacteria or parasites and at tumor cells. Cytotoxic T cells also attack foreign cells that are present in blood transfusions and tissue transplants. People with organ trans-plants (such as heart transplants) must take immunosuppressive drugs to sup-press the cytotoxic T cells and allow the transplanted organs to survive.