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The thymus is essential to the maturation of the immune system.
It contributes to the development of T lymphocytes, guard cells
that participate in the immunoreactions. Unfortunately, the thymus
atrophies with age, and the production of defense cells gradually
decline, making human beings more vulnerable to various diseases.
Thymus: An Essential Gland of the Immune System
The thymus is essential to the maturation and maintenance of the
cells of the immune system. It exerts its influence by liberating
hormonal factors that act inside the thymus itself as well as elsewhere
in the body. Blood concentrations of these thymic factors reach
their maximal value at puberty, and gradually decline afterwards.
Immune system deficiencies observed in old people and in several
diseases are linked in good part to the abnormally low level of
thymic factors.
The immune system's function is to protect the body against parasitic
infections (viruses, bacteria, fungi, microbes, etc.) that cause
invasive and sometimes fatal diseases. The immune system uses, among
other things, the lymphocytes. Many types of lymphocytes participate
to the immune response (Figure 1). For example, during a parasitic
infection, the lymphocytes called macrophages phagocytize the parasites
and break them up into fragments which are then exposed at the surface
of the said macrophage and presented to the immunoregulator T lymphocytes.
Those T lymphocytes activate and release messenger substances (cytokines)
that increase the immune response and stimulate the production of
antibodies by B lymphocytes. Other types of T lymphocytes have a
direct cytotoxic action on cells presenting parasite fragments,
thus completing the immune response.
At birth, the child benefits from the lymphocytes passed on to
him by his mother, which protect him from infections. He must quickly
start to produce his own lymphocytes, which will mostly originate
from bone marrow and thymus. A thymic deficiency in the newborn
will result in lack of T lymphocytes and in the quick appearance
of physiological complications that will translate into slowed growth
rate or repetitive infections, and possibly lead to premature death.
Maturation of T Lymphocytes: An Important Event in the
Immune Response
The maturation and proliferation of T lymphocytes are regulated
by factors produced and released by the thymus and the lymphocytes
themselves. A large number of distinct factors are necessary to
maintain a balanced production between the various types of T lymphocytes;
these factors include almost all interleukins, thymosins, thymopoietin,
thymic humoral factor, thymic factor X, serum thymic factor, as
well as other thymic factors not as well characterized. These serum
factors have a direct effect on the maturation and differentiation
of the various types of T lymphocytes.
The lymphocytes of the thymus are multipotential cells,
which means they can undergo several differentiation pathways to
ultimately become functional T lymphocytes (Figure 2). Those T lymphocytes
include the effector (cytotoxic cells) and immuno-regulator (auxiliary
cells, suppressive cells) lymphocytes. When a T lymphocyte is activated
by a parasite, it commits itself and becomes specific to this parasite,
which will thereafter be the only activator of this sensitized lymphocyte.
The committed T lymphocyte stays vigilant for the rest of its life;
its survival depends on the presence of thymic factors. In a way,
the committed T lymphocytes constitute a major constituent of the
immune memory, allowing the body to quickly react when exposed again
to the parasite.
Involution of the Thymus During Aging and Sensitivity
to Infections
The thymic gland is located at the base of the neck. In humans,
it continues to develop after birth, and reaches its maximal size
at puberty (approximately 60 g). Both lobes of the thymus are divided
into lobules, which contain a cortex and a medulla. The cortex is
composed almost essentially of lymphocytes, that are the actual
defense cells, while the medulla is mostly made of epithelial cells
with a few scattered lymphocytes. The epithelial cells of the thymus
produce most of the thymic factors necessary for the maturation
and maintenance of the immune cells.
During aging, the thymus gradually decreases in size and activity
(Figure 3). Its weight decreases by 2/3 and its lymphocyte content
by 90%. Cell death occurring in the tissue is not caused by the
disappearance of any essential substance; in fact, this involutional
phase is still not well understood, and it is believed to be a natural,
genetically-programmed phenomenon.
The serum concentrations of thymic hormones also decrease after
puberty and reach their lowest value at age 60 and on. This decrease
clearly shows the deterioration of the immune system's competence
observed in old people.
Conclusion: The thymus is an essential organ of
the immune system. In the adult, its functions are to produce mature
lymphocytes and to maintain the health of the surveillance lymphocytes
that circulate in the body and stay alert in order to quickly defend
the body against a parasitic intrusion. Thymic factor deficiencies
caused by illness or aging are responsible for several immune system
deficiencies; infections are then more difficult to fight, and they
gradually drain the vital energy of the infected individual.
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