T cells differ from B lymphocytes in an important way. T cells, responsible for dealing with most viruses, some bacteria and fungi, and for cancer surveillance, are one of the two main classes of lymphocytes. As one variety of white blood cells, lymphocytes are part of the body's immune system; the immune system fights invading organisms that have penetrated the body's general defenses.
T lymphocytes, or T cells, compose about 80 percent of the lymphocytes circulating in the blood. They have been educated in the thymus to perform particular functions. Killer T cells are sensitized to multiply when they come into contact with antigens (foreign proteins) on abnormal body cells (cells that have been invaded by viruses, cells in transplanted tissue or tumor cells).
These killer T cells attach themselves to the abnormal cells and release chemicals (lymphokines) to destroy them. Helper T cells assist killer T cells in their activities and control other aspects of the immune response.
When B lymphocytes, which compose about 10 percent of total lymphocytes, contact the antigens on abnormal cells, the lymphocytes enlarge and divide to become plasma cells. The plasma cells then secrete vast numbers of immunoglobulins, or antibodies, into the blood that attach themselves to the surfaces of the abnormal cells, beginning a process leading to the destruction of the invaders.