bin production in some children). BONE MARROW
TRANSPLANTATIONis sometimes an option for people
who have severe symptoms. There is no cure for
sickle cell disease. Sickle cell trait does not pro-
duce symptoms or develop into sickle cell disease,
so it requires no treatment. Many people who
have sickle cell trait do not know it.
Risk Factors and Preventive Measures
Because sickle cell disease is an inherited genetic
disorder, the only risk factor is heredity. It is a
good idea for people who do not know their sickle
cell status, especially African Americans, to have
the blood test for hemoglobin S before conceiving
children. GENETIC COUNSELINGcan help with family
planning decisions when both parents have sickle
cell trait.
See also BLOOD TRANSFUSION; HEMOLYSIS; PRIAPISM.
spleen A soft structure of lymphatic tissue
located in the upper left abdomen to the left of the
STOMACH and PANCREAS, behind the protective
enclosure of the rib cage. Fibrous ligaments
anchor the spleen to the stomach, COLON, and left
kidney. The spleen holds about 300 milliliters of
BLOOD, roughly 4 percent of the body’s total blood
supply, and contains about a third of the body’s
platelets (the cells responsible for COAGULATION). Its
high blood content gives the spleen a dark red
color and a somewhat porous texture. The spleen
has two main structural and functional sections
that filter the blood for different substances, the
red pulp and the white pulp.
White Pulp
The white pulp consists of nodules and follicles,
similar to those of other lymphatic tissues such as
the LY M P Hnodes, arranged in sheathlike structures
that encase each of the tiny blood vessels (arteri-
oles) within the spleen. The white pulp has two
primary roles, to filter antigens from the circulat-
ing blood and to produce lymphocytes (a type of
LEUKOCYTE). These functions are interrelated in
that the lymphocytes bear antibodies specific to
the antigens the white pulp traps. When the lym-
phocytes enter the circulation of the blood or
lymph, their antibodies allow them to intercept
and destroy pathogens such as viruses or BACTERIA
that carry the antigens.
Red Pulp
The red pulp surrounds the white pulp. In the
developing fetus the red pulp produces the major-
ity of blood cells, erythrocytes (red blood cells)
and leukocytes (white blood cells) alike, until
about the fifth month of PREGNANCY, after which
the red bone marrow takes over erythropoiesis
(ERYTHROCYTEproduction). Hematopoietic capabil-
ity of the red pulp remains available but dormant
after birth. Throughout life, the red pulp serves as
an extramedullary (out of the marrow) resource
that the body can press into action to produce ery-
throcytes.
The red pulp also filters the blood, culling out-
dated, defective, or damaged erythrocytes from cir-
culation. Phagocytic cells called macrophages that
reside within the red pulp break down the erythro-
cytes, sending components such as HEMOGLOBINand
BILIRUBINback into the blood for transportation to
the LIVER, which recycles them. The red pulp also
filters other cellular debris from the blood.Potential Health Conditions Involving the Spleen
The spleen’s primary vulnerability is trauma,
which can cause life-threatening hemorrhage
(uncontrolled bleeding). A blow to the upper
abdomen, such as may occur in MOTOR VEHICLE
ACCIDENTSor with aggressive contact sports, can
cause a rupture injury to the spleen. Such a blow
also can fracture a rib, causing a penetrating
wound to the spleen. SPLENECTOMY (surgical
removal of the spleen) then becomes necessary.
Though not essential for life, the spleen performs
numerous functions vital to the IMMUNE RESPONSE
and blood cell maintenance. Other lymphatic
structures and the liver can partially compensate
for the spleen’s loss, though the risk for serious
INFECTIONsignificantly increases. There are many
health conditions that can cause the spleen to
enlarge (SPLENOMEGALY). The spleen also enlarges
when fighting systemic infections such as infec-
tious mononucleosis and in some cancers.
For further discussion of the spleen within the
context of blood and lymph structure and func-
tion please see the overview section “The Blood
and Lymph.”
See also CANCER; LYMPH NODE; MONONUCLEAR
PHAGOCYTE SYSTEM; MONONUCLEOSIS, INFECTIOUS;
PHAGOCYTOSIS.spleen 167