244
U N I T 3
Hematopoietic Function
granules.
The azurophilic granules stain purple and are
lysosomes. The granulocytes are divided into three types—
neutrophils, eosinophils, and basophils—according to
the staining properties of their specific granules.
Neutrophils.
The neutrophils, which constitute 60% to
65% of the total white blood cells, have specific gran-
ules that are neutral and do not stain with an acidic or
a basic dye.
1
Because these white cells have nuclei that
are divided into three to five lobes, they are often called
polymorphonuclear leukocytes
(PMNs).
The neutrophils are primarily responsible for main-
taining normal host defenses against invading bacteria
and fungi, cell debris, and a variety of foreign sub-
stances. After release from the marrow, the neutrophils
spend only approximately 4 to 8 hours in the circula-
tion before moving into the tissues. They survive in
the tissues for approximately 4 to 5 days.
1
They die in
the tissues by discharging their phagocytic function or
from senescence. The pool of circulating neutrophils
(i.e., those that appear in the blood count) is in a closely
maintained equilibrium with a similar-sized pool of
cells marginating along the walls of small blood vessels.
These are the neutrophils that respond to chemotactic
factors and migrate into the tissues toward the offend-
ing agent during an inflammatory reaction.
Eosinophils.
The specific cytoplasmic granules of the
eosinophils stain red with the acidic dye eosin. These
leukocytes constitute 1% to 3% of the total white blood
cells and increase in number during allergic reactions
and parasitic infections.
1
During allergic reactions, they
are thought to release enzymes or chemical mediators
that detoxify agents associated with the reaction. In
parasitic infections, the eosinophils use surface mark-
ers to attach themselves to the parasite and then release
hydrolytic enzymes that kill it.
Basophils.
The basophils are the least numerous of the
white blood cells, accounting for only 0.3% to 0.5% of
the total leukocytes.
1
The specific granules of the baso-
phils stain blue with a basic dye. These granules contain
heparin, an anticoagulant; histamine, a vasodilator; and
other mediators of inflammation. The basophil, which
is a blood cell, is related to the connective tissue mast
cell that contains similar granules. Both the basophils
and mast cells are thought to be involved in allergic and
hypersensitivity reactions.
Agranulocytes
Agranulocytes are leukocytes that lack cytoplasmic gran-
ules. They include both the lymphocytes and monocytes/
macrophages.
Lymphocytes.
Lymphocytes are the most common of
the agranulocytes, accounting for approximately 30%
of the total blood leukocytes.
1
They originate in the bone
marrow from lymphoid stem cells and migrate through
the peripheral lymphoid organs, where they recognize
antigens and participate in immune responses.
There are three types of lymphocytes: B lympho-
cytes, T lymphocytes, and natural killer cells.
1,2
The
B lymphocytes
are so named because they were first
recognized as a separate population in the bursa of
Fabricius in birds and bursa-equivalent organs (e.g.,
bone marrow) in mammals. They differentiate to form
antibody-producing plasma cells and are involved in
humoral-mediated immunity. The
T lymphocytes
dif-
ferentiate in the thymus. They activate other cells of the
immune system (helper T cells) and are involved in cell-
mediated immunity (cytotoxic T cells).
Natural killer
cells
participate in innate or natural immunity and their
function is to destroy foreign cells. The lymphocytes of
the three different subsets have unique surface markers
that can be identified and used to define their function
and diagnose disease (discussed in Chapter 15).
Although all lymphocytes are morphologically simi-
lar, they comprise elements that vary in terms of lineage,
cell membrane molecules and receptors, function, and
response to antigen. These cells are often distinguished
by surface proteins that can be identified using panels
of monoclonal antibodies. These identified proteins
are then correlated with cell functions. The standard
nomenclature for these proteins is the “CD” (clusters
of differentiation) numeric designation (CD4, CD8),
which is used to delineate surface proteins that define
a particular cell type or stage of cell differentiation and
are recognized by a cluster or group of antibodies.
2
Although this nomenclature was originally developed
for lymphocytes, it is now common practice to apply it
to blood cells other than lymphocytes.
Monocytes/Macrophages.
Monocytes are the largest
of the white blood cells and constitute approximately
3% to 8% of the total leukocyte count. They are dis-
tinguished by a large amount of cytoplasm and a darkly
stained kidney-shaped nucleus. Although these cells are
considered agranular, they contain small, dense, azuro-
philic granules that contain lysosomal enzymes similar
to those found in the azurophilic granules of neutro-
phils. Monocytes travel from the bone marrow to the
body tissues, where they differentiate into various tissue
phagocytes including the
histiocytes
of loose connective
tissue,
microglial cells
of the brain,
Kupffer cells
of the
liver, and tissue macrophages. During inflammation,
monocytes leave the blood vessel at the site of inflamma-
tion and transform into tissue macrophages that phago-
cytose bacteria and tissue debris. Macrophages also
play an important role in immune responses by activat-
ing lymphocytes and by presenting antigen to T cells
(see Chapter 15).
Leukocyte Developmental Stages
Leukocyte development begins with the myeloid and lym-
phoid stem cells in the bone marrow
2–4
(Fig. 11-3). The
immature precursor cells for each of the cell lines are called
blast cells.
The names of the various leukocyte devel-
opmental stages are often used in describing blood cell
changes that occur in hematopoietic disorders (e.g., acute
lymphoblastic leukemia, chronic myelogenous leukemia).
