LYMPHOCYTE

type of leukocyte (white blood cell) that is of fundamental importance in the immune system. In humans lymphocytes make up 25 to 33 percent of the total number of leukocytes. They are found in the circulation and also are concentrated in lymphoid organs and tissues, such as the spleen, thymus, tonsils, and lymph nodes, where they are most likely to encounter infectious microorganisms. The two primary types of lymphocytes are B lymphocytes and T lymphocytes, or B cells and T cells. Both originate from stem cells in the bone marrow and are initially similar in appearance. Some lymphocytes migrate to the thymus, where they mature into T cells; others remain in the bone marrow, wherein humansthey develop into B cells. Most T cells are long-lived, their life span averaging two to four years, while most B cells are short-lived, with an average life span of a week to a few months. The basic structure of a typical T-cell antigen receptor. Through receptor molecules on their surfaces, lymphocytes are able to bind to antigens (foreign substances or microorganisms that the host recognizes as nonself) and help remove them from the body. Each lymphocyte bears receptors that bind to a specific antigen. The ability to respond to virtually any antigen comes from the fact that the body contains an enormous variety of lymphocyte populations, each of which is capable of recognizing a unique antigen. Clonal selection of a B cell Activated by the binding of an antigen to a Once stimulated by binding to a foreign antigen, such as a component of a bacterium or virus, a B cell multiplies into a clone of identical cells. Some of these cloned B cells differentiate into plasma cells that produce antibody molecules. These antibodies are closely modeled after the receptors of the precursor B cell, and, once released into the blood and lymph, they bind to the target antigen and initiate its neutralization or destruction. Antibody production continues for several days, until the antigen has been overcome. Other B cells, called memory cells, are stimulated to multiply but do not differentiate into plasma cells; they provide the immune system with long-lasting memory and a heightened sensitivity to a particular type of antigen should it return to the host. Stimulation of immune response by activated helper T cells Activated by In the thymus, T cells multiply and differentiate into helper, suppressor, and cytotoxic T cells. Once stimulated by the appropriate antigen, helper T cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells, thereby promoting antibody production. Suppressor T cells act to diminish this reaction, hence their name. Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.