macrophages that phagocytize old RBCs) and in the
lymph nodes. They phagocytize pathogens that circu-
late in blood or lymph through these organs. Other
“wandering” macrophages move about in tissue fluid,
especially in the areolar connective tissue of mucous
membranes and below the skin. Pathogens that gain
entry into the body through natural openings or
through breaks in the skin are usually destroyed by the
macrophages and other leukocytes in connective tissue
before they can cause serious disease. The alveoli
of the lungs, for example, have macrophages that
very efficiently destroy pathogens that enter with
inhaled air.
A high WBC count, called leukocytosis, is often
an indication of infection. Leukopeniais a low WBC
count, which may be present in the early stages of dis-
eases such as tuberculosis. Exposure to radiation or to
chemicals such as benzene may destroy WBCs and
lower the total count. Such a person is then very sus-
ceptible to infection. Leukemia, or malignancy of
leukocyte-forming tissues, is discussed in Box 11–4:
Leukemia.
The white blood cell types (analogous to RBC types
such as the ABO group) are called human leukocyte
antigens (HLA)and are discussed in Box 11–5: White
Blood Cell Types: HLA.PLATELETS
The more formal name for platelets is thrombocytes,
which are not whole cells but rather fragments or
pieces of cells. Some of the stem cells in the red bone
marrow differentiate into large cells called megakary-
ocytes(see Figs. 11–2 and 11–3), which break up into
small pieces that enter circulation. These small, oval,
circulating pieces are platelets, which may last for 5 to
9 days, if not utilized before that. Thrombopoietinis
a hormone produced by the liver that increases the
rate of platelet production.
A normal platelet count (part of a CBC) is 150,000
to 300,000/L (the high end of the range may be
extended to 500,000). Thrombocytopeniais the term
for a low platelet count.Function
Platelets are necessary for hemostasis, which means
prevention of blood loss. There are three mechanisms,Blood 263BOX11–4 LEUKEMIA
Chemotherapy may bring about cure or remis-
sion for some forms of leukemia, but other forms
remain resistant to treatment and may be fatal
within a few months of diagnosis. In such cases, the
cause of death is often pneumonia or some other
serious infection, because the abnormal white
blood cells cannot prevent the growth and spread
of pathogens within the body.Leukemiais the term for malignancy of the blood-
forming tissue. There are many types of leukemia,
which are classified as acute or chronic, by the
types of abnormal cells produced, and by either
childhood or adult onset.
In general, leukemia is characterized by an over-
production of immature white blood cells. These
immature cells cannot perform their normal func-
tions, and the person becomes very susceptible to
infection. As a greater proportion of the body’s
nutrients are used by malignant cells, the produc-
tion of other blood cells decreases. Severe anemia is
a consequence of decreased red blood cell produc-
tion, and the tendency to hemorrhage is the result
of decreased platelets.Box Figure 11–B Leukemia. Notice the many
darkly staining WBCs (300). (From Sacher, RA, and
McPherson, RA: Widmann’s Clinical Interpretation
of Laboratory Tests, ed 11. FA Davis, Philadelphia,
2000, with permission.)