Encyclopedia of Health and Medicine

The Blood and Lymph 121
appears the education of T-cells somehow goes
awry, and the thymus releases those that identify
certain self cells as nonself. These misguided T
cells attack the mistaken self cells as though they
were nonself, causing an autoimmune (selfattack)
response.
The thymus is most active in childhood, reaching
its peak function and size in early ADOLESCENCE.
By early adulthood the thymus shrinks to a mere
fibrous shadow of its most proliferative self. In the
1940s doctors erroneously drew a correlation
between the normally large thymus of childhood
and what subsequently became known as SUDDEN
INFANT DEATH SYNDROME (SIDS), resulting in “therapeutic”
irradiation of the thymus to reduce its size.
Unfortunately, this permanently crippled the
immune system, an often fatal consequence in a
time when antibiotic therapy was in its infancy.
Though even today researchers do not fully
understand the role of the thymus in adulthood,
they know it secretes a number of hormones that
appear to have functions related to immune
response. THYMECTOMY (surgery to remove the thymus)
remains a therapeutic option in very limited
circumstances, such as in adults who have myasthenia
gravis.
Health and Disorders of the Blood
The blood often is the first location within the
body where health conditions manifest, and as
well is itself vulnerable to disorders that affect its
ability to function. Even conditions that do not
directly affect the blood’s function show up in the
blood, such as DIABETES (elevated blood glucose
levels) and ATHEROSCLEROSIS (elevated blood lipids).
Diagnostic blood tests, notably the complete blood
count (CBC), are part of most clinical evaluations.
The numbers, types, and cytologic details of the
blood cells provide crucial clues to doctors investigating
broad-ranging symptoms such as fatigue,
chronic infection, or allergies.
Anemia, an inability of the blood to meet the
body’s oxygenation needs that affects about 3.5
million people in the United States, results from
numerous and varied health circumstances and
conditions. SICKLE CELL DISEASE and THALASSEMIA are
GENETIC DISORDERS that result in defective erythrocytes.
LEUKEMIA, lymphoma, and MULTIPLE MYELOMA
are cancers that involve the blood cells and the
structures that make them. Inherited deficiencies
alter specific aspects of the blood’s composition
and function, such as HEMOPHILIA, a deficiency of
clotting factors that results in excessive bleeding.
In many situations treating the underlying health
condition eliminates its effect on the blood, such
as with many types of anemia. In other cases,
such as leukemia, treatment targets the blood or
blood-producing organs and structures.
CONDITIONS THAT INVOLVE THE BLOOD
AND THE LYMPH
ANEMIA
hemangioma
HEMOPHILIA
LEUKOPENIA
LYMPHEDEMA
lymphoma
MULTIPLE MYELOMA
MYELOFIBROSIS
POLYCYTHEMIA VERA
THROMBOCYTHEMIA
thrombophilia
VON WILLEBRAND’S DISEASE
DISSEMINATED INTRAVASCULAR
COAGULATION (DIC)
HEMOCHROMATOSIS
LEUKEMIA
LYMPHADENITIS
lymphocytopenia
methemoglobinemia
MYELODYSPLASIA SYNDROME
NEUTROPENIA
THALASSEMIA
THROMBOCYTOPENIA
thrombosis
Traditions in Medical History
Though ancient healers understood the importance
of blood to health and to life itself, they did
not understand the mechanisms of its circulation
or production. Doctors did not know these details
of physiology until the 17th and 18th centuries,
respectively. The great GALEN (129–199), father of
Western medicine, pronounced that the liver was
the source of the body’s blood, constantly producing
this vital fluid much as a natural spring produced
water, with an equal mix of regularity and
mystery. Though centuries later researchers would
discover the fragment of truth in this view—the
liver does indeed produce the cells of the blood
early in fetal development—its fallacy nurtured
peculiar medical practices throughout much of
modern history. Among the most persistent was
that of bloodletting, which remained a mainstay of
clinical practice into the 20th century as a treatment
for nearly any condition that did not
respond to other therapeutic methods. Modern
doctors know, of course, that bloodletting drains
the body of the cells it needs most to fight infection.