-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).
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.