therapy may also be appropriate for people who
cannot take methylene blue.
Recovery is generally complete when the cause
is toxic exposure. Genetic disorders of the hemo-
globin or the enzyme mechanisms that regulate
the balance between methemoglobin and hemo-
globin may result in chronic methemoglobinemia
and consequently the need for ongoing treatment
(such as oral methylene blue) to prevent toxic
accumulations.
Risk Factors and Preventive MeasuresThe most common cause of methemoglobinemia
is toxic exposure to substances that cause oxida-
tion, which overwhelms the body’s normal mech-
anisms for managing cell metabolism. Avoiding
chemicals, including drugs, that may cause methe-
moglobinemia is not simple, as they number in
the dozens and include such commonly used
medications as nitrates (cardiovascular) and topi-
cal anesthetics. People who have genetic disorders
that interfere with hemoglobin production and
function have increased risk for methemoglobine-
mia and should make every effort to avoid known
causative agents.
See also ENVIRONMENTAL HAZARD EXPOSURE; G 6 PD
DEFICIENCY; OCCUPATIONAL HEALTH AND SAFETY; OXY-
GEN–CARBON DIOXIDE EXCHANGE; SICKLE CELL DISEASE.
monocyte A LEUKOCYTE(white BLOODcell), also
called an agranulocyte, that has a single-lobed
nucleus and contains no granules in its cytoplasm.
The BONE MARROWand the LY M P Hnodes produce
monocytes, which have a two-phase existence in
the body. During the first phase, the monocyte cir-
culates in the blood and the lymph, functioning as
a PHAGOCYTEthat consumes pathogenic particles in
the circulation. After about 24 hours the mono-
cyte migrates into the tissue to enter its second
phase of life. Once in the tissue the monocyte
matures, becoming a fixed phagocytic cell called a
macrophage that may acquire a specific name,
depending on its location. About half of the body’s
macrophages migrate to the lymphatic structures.
Most of the remainder reside in the LIVER,
where they are called Kupffer cells. Macrophages
that settle in the layers of the SKINare Langerhans
cells, and those that inhabit the BONEare osteo-
clasts.
Two to 8 percent of the body’s leukocytes are
monocytes; a normal monocyte count is 200 to
1100 monocytes per microliter of whole blood.
The number of monocytes in circulation may
increase with INFECTION, LEUKEMIA, LYMPHOMA, many
other types of cancer, and AUTOIMMUNE DISORDERS
in which there is active INFLAMMATIONand autoim-
mune activity. The number of monocytes in circu-
lation may decrease in aplastic ANEMIAand with
steroid medications.
For further discussion of monocytes within the
context of blood and lymph structure and func-
tion please see the overview section “The Blood
and Lymph.”
See also CELL STRUCTURE AND FUNCTION; ERYTHRO-
CYTE; GRANULOCYTE; HEMATOPOIESIS; SKIN-ASSOCIATED
LYMPHOID TISSUE(SALT).multiple myeloma A CANCERof the BONE MARROW
in which PLASMAcells, also called myeloma cells or
myelocytes, proliferate, accumulating as lesions
(growths) to develop within the BONEmarrow cav-
ities of the bones. The lesions prevent normal
functioning of the bone marrow. They also dam-
age bone tissue and weaken the bone structure.
Plasma cells derive from lymphocytes that migrate
to the bone marrow. Their function is to produce
immune antibodies, or immunoglobulins, that are
essential for the body’s IMMUNE RESPONSE. They
generally make up less than 5 percent of the cells
in the bone marrow. In multiple myeloma plasma
cells make up 10 percent or more of the bone
marrow’s cells. The cancerous plasma cells of mul-
tiple myeloma overproduce certain immune anti-
bodies called monoclonal proteins or M-proteins.
M-proteins alter the ways in which immunoglob-
ulins bind with B-cell lymphocytes in the blood,
reducing their ability to fight INFECTION.
The M-proteins also activate specialized phago-
cytic cells in the bone, called osteoclasts, accelerat-
ing the deconstruction phase of bone remodeling
(the process through which bone tissue continu-
ously replenishes). Osteoclastic activity releases
excessive calcium into the bloodstream, affecting
numerous body systems, including cardiovascular
function and renal (kidney) function. The KIDNEYS
produce ERYTHROPOIETIN(EPO), the HORMONE that
stimulates the bone marrow to produce erythro-
cytes (red blood cells). Damage to the kidneysmultiple myeloma 157