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The immune system is comprised of specialized white blood cells,
called lymphocytes that adapt themselves to fight specific foreign
invaders. These cells develop into two groups in the bone
marrow.
From the bone marrow, one group of lymphocytes migrates to a gland
called the thymus and become T lymphocytes or T cells. Within the
thymus, the T cells mature under the influence of several
hormones.
The T cells mature into several different types, including helper,
killer and suppressor cells. When matured, the T cell types are
ready to work together to directly attack foreign invaders,
providing what physicians call cell-mediated immunity. This type of
immunity can become deficient in persons with HIV, the virus that
causes AIDS, because HIV attacks and destroys helper T cells.
The other group of lymphocytes, B lymphocytes or B cells, mature
and develop within the bone marrow itself. In that process, they
achieve the ability to recognize specific foreign invaders. From
the bone marrow, B cells migrate through the body fluids to the
lymph nodes, spleen and blood. B lymphocytes provide the body with
humoral immunity as they circulate in the fluids in search of
specific foreign invaders to destroy.
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