The immune system includes 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. T cells
are responsible for cell-mediated immunity. This type of
immunity becomes 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