Leukocytes, erythrocytes, and blood platelets are all ultimately derived from ("stem from") unspecialized cells in the bone marrow. These stem cells produce the specialized blood cells, and they replace themselves by cell division so that the stem cell population is not exhausted. Lymphocytes produced in this manner seed the thymus, spleen, and lymph nodes, producing self-replacing lymphocyte colonies in these organs.
The lymphocytes that seed the thymus become T lymphocytes, or T cells (the letter T stands for thymus-dependent). These cells have surface characteristics and an immunological function that differ from those of other lymphocytes. The thymus, in turn, seeds other organs; about 65% to 85% of the lymphocytes in blood and most of the lymphocytes in the germinal centers of the lymph nodes and spleen are T lymphocytes. T lymphocytes, therefore, either come from or had an ancestor that came from the thymus.
Most of the lymphocytes that are not T lymphocytes are called B lymphocytes, or B cells. The letter B derives from immunological research performed in chickens. Chickens have an organ called the bursa of Fabricius that processes B lymphocytes. Since mammals do not have a bursa, the B is often translated as the "bursa equivalent" for humans and other mammals. It is currently believed that the B lymphocytes in mammals are processed in the bone marrow, which conveniently also begins with the letter B. Since the bone marrow produces B lymphocytes and the thymus produces T lymphocytes, the bone marrow and thymus are considered to be primary lymphoid organs.
Both B and T lymphocytes function in specific immunity. The B lymphocytes combat bacterial infections, as well as some viral infections, by secreting antibodies into the blood and lymph. Because blood and lymph are body fluids (humors), the B lymphocytes are said to provide humoral immunity, although the term antibody-mediated immunity is also used. T lymphocytes attack host cells that have become infected with viruses or fungi, transplanted human cells, and cancerous cells. The T lymphocytes do not secrete antibodies; they must come in close proximity to the victim cell, or have actual physical contact with the cell, in order to destroy it. T lymphocytes are therefore said to provide cell-mediated immunity (table 15.4).
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