388 MEDICINAL CHEMISTRY
The initial distinction is between granular and agranular leukocytes, depending upon
whether they contain small inclusions, called granules, as seen at the level of the light
microscope.
Since the leukocytes are the “foot soldiers” of the immune system, they have a
number of “training camps and bases of operation.” The bone marrow appears to be the
source of stem cells for all of the cellular elements of blood. A single pluripotential
stem cellserves as a precursor for all leukocytes, both lymphocytes and granulocytes.
Thethymus,a small organ located just behind the sternum, is responsible for the matu-
ration and development of T lymphocytes; it also exerts a measure of control and main-
tenance over other activities of the immune system. The thymus is sometimes removed
to control an unusual immune-mediated disease called myasthenia gravis.Thespleen,
located in the upper left quadrant of the abdomen, has diffusely packed areas of T cells
and B cells and serves as a major filter for blood-borne antigens. Since the spleen is
vulnerable to rupture during abdominal trauma, it is sometimes surgically removed
(splenectomy); people without a spleen are somewhat more susceptible to certain types
of bacterial infections. Lymph nodesare regionalized collections of lymphocytes and
macrophages. Enlarged lymph nodes may represent a response to a localized infection
(“swollen glands” in the neck during a bacteria-induced “sore throat”) or regional
spread of a cancer (firm, enlarged lymph nodes in the armpit of a woman with breast
cancer). (See figure 6.1.)
Figure 6.1 Relevant anatomy of the immune system. The immune system is distributed through-
out the body, permitting it to fulfill its goal of maintaining structural homeostasis in the body
at a cellular level. A number of immunomodulator molecules are the messengers of the immune
system.