dispenses the likelihood for developing thrombo-
cytopenia.
See also DISSEMINATED INTRAVASCULAR COAGULATION
(DIC); THROMBOCYTHEMIA.
thymectomy A surgical OPERATION, performed
under general ANESTHESIA, to remove the THYMUS, a
structure of lymphatic tissue located behind the
sternum (breastbone) that produces T-cells. Ten-
drils of the thymus often extend upward toward
the THYROID GLANDand downward over the HEART.
The loose structure of the thymus can make it
challenging to surgically remove.
Thymectomy is the treatment of choice for
most cancers of the thymus, which are uncom-
mon. Doctors sometimes use thymectomy to treat
severe AUTOIMMUNE DISORDERS, such as MYASTHENIA
GRAVIS, as a method to restrict the body’s ability to
produce T-cells and thus limit the IMMUNE
RESPONSE. Thymectomy in childhood has extensive
consequences for IMMUNE SYSTEM development
though does not appear to alter IMMUNE RESPONSE
in adults. Most people require only a one or two
day stay in the hospital, and about four to six
weeks for full recovery.
See alsoLYMPHOCYTE; SPLENECTOMY.
thymus A structure of lymphatic tissue located
in the upper central chest, behind the sternum
(breastbone) midway between top of the HEART
and the sternal notch at the base of the THROAT.
The thymus is fairly large at birth, spreading in a
loosely shaped “H” across the great vessels that
emerge from the heart. The tissue of the thymus
sometimes extends upward to make contact with
the THYROID GLANDand downward to drape over
the heart’s upper chambers, the atria. Occasionally
fragments of thymic tissue exist as unattached,
isolated lobules typically remaining in the upper
chest and lower neck.
The thymus is most active in youth, serving as
the incubator in which T-cells, the lymphocytes
crucial for the body’s defense against INFECTION,
mature and differentiate (acquire their functional
characteristics). The thymus typically enlarges as a
child approaches PUBERTY, its peak time of activity,
then begins to recede. By midlife little more than
strands of thymic tissue remain. The thymus also
produces hormones—key among them being thy-
mosin, thymulin, thymopoietin, and thymocyte
humoral factor—that regulate T-cell maturation.STATUS LYMPHATICUS: “ENLARGED” THYMUS
In the 1940s and 1950s, conventional medical
wisdom blamed the large thymus of childhood
for unexplained sudden death in children, con-
veying upon the condition the diagnostic label
status lymphaticus.RADIATION THERAPYto shrink the
thymus became the prevailing treatment. By the
1960s doctors recognized the thymus was nor-
mally large in children and abandoned the diag-
nosis and its treatment.Researchers believe the thymus has other func-
tions related to IMMUNE RESPONSE, though remain
unable to determine their precise mechanisms. An
infant born without a thymus has no ability to
develop an IMMUNE SYSTEM; this CONGENITAL ANOM-
ALYis nearly always fatal in infancy. There also
appears to be a correlation between the thymus
and MYASTHENIA GRAVIS, an autoimmune disorder in
which the immune system produces antibodies
that target acetylcholine, the NEUROTRANSMITTER
that facilitates NERVE signals traveling between
nerve cells and MUSCLEcells. An enlarged thymus
is common in people who have myasthenia gravis
and removing it (THYMECTOMY) often results in dra-
matic improvement in the condition. The thymus
may have a role in other AUTOIMMUNE DISORDERS
such as SYSTEMIC LUPUS ERYTHEMATOSUS(SLE) and
GRAVES’S DISEASE. Thymoma and thymic CARCINOMA
are two forms of cancer that may develop in the
thymus. Cancer of the thymus is uncommon.
For further discussion of the thymus within the
context of blood and lymph structure and func-
tion please see the overview section “The Blood
and Lymph.”
See alsoHIV/AIDS.172 The Blood and Lymph