246
U N I T 3
Hematopoietic Function
The alimentary canal, respiratory passages, and
genitourinary systems are guarded by accumulations
of lymphoid tissue that are not enclosed in a capsule.
This form of lymphoid tissue is called
diffuse lymphoid
tissue
or
mucosa-associated lymphoid tissue
(MALT)
because of its association with mucous membranes.
Lymphocytes are found in the subepithelium of these tis-
sues. Lymphomas can arise from either MALT or lymph
node tissue.
Nonneoplastic Disorders
of White Blood Cells
The number of leukocytes, or white blood cells, in the
peripheral circulation normally ranges from 4500 to
10,500 cells/
μ
L (4.5 to 10.5 × 10
9
/L).
6
The nonneoplas-
tic disorders of white blood cells include a deficiency of
leukocytes (i.e., leukopenia) or proliferation of excess
white blood cells (i.e., leukocytosis).
Neutropenia (Agranulocytosis)
The term
leukopenia
describes a decrease in the absolute
number of leukocytes in the blood. The disorder may
affect any of the specific types of white blood cells, but
most often affects the neutrophils, which are the pre-
dominant type of granulocyte.
Agranulocytosis
denotes
a virtual absence of neutrophils. In
aplastic anemia,
all
of the myeloid stem cells are affected, resulting in ane-
mia, thrombocytopenia, and agranulocytosis.
Neutropenia
refers specifically to an abnormally low
number of neutrophils and is commonly defined as a
circulating neutrophil count of less than 1500/
μ
L.
4,7–11
It
can be further graded as mild (1000 to 1500/
μ
L), mod-
erate (500 to 1000/
μ
L), or severe (<500/
μ
L) based on an
absolute number of neutrophils circulating in the blood.
Since the neutrophil protects against bacterial infec-
tions, persons with neutropenia are prone to recurrent
and sometimes severe bacterial infections.
Individuals of African descent and some ethnic
groups from the Middle East have lower neutrophil
counts without predisposition to bacterial infection,
a condition referred to as
benign ethnic neutropenia
.
11
Secondary follicle
Superficial cortex (B cell zone)
Germinal center
Mantle zone
Primary follicle
Afferent lymphatic
Trabecula
Capsule
Artery
Paracortex
(T cell zone)
Vein
Efferent lymphatic
FIGURE 11-4.
Structures of normal lymph node.
SUMMARY CONCEPTS
■■
The hematopoietic system consists of the
different types of blood cells generated from
the pluripotent stem cells in the bone marrow.
These stem cells differentiate into committed cell
lines that develop into red blood cells, platelets,
and leukocytes.The development of the different
types of blood cells is supported by chemical
messengers, called colony-stimulating factors,
other growth factors, and chemical mediators.
■■
White blood cells (leukocytes) development
begins with myeloid stem cells that develop
into granulocyte and monocyte cell lines, and
lymphoid stem cells that develop into the
lymphocyte cell line.The immature precursor
cells for each of the cell lines are called blast
cells.The blast cells progress through various
maturational stages before becoming mature
granulocytes, monocytes, or lymphocytes.
■■
The life span of white blood cells is relatively
short so that constant renewal is necessary to
maintain normal blood levels. Any conditions
that decrease the availability of stem cells or
hematopoietic growth factors produce a decrease
in white blood cells.
■■
The lymphatic system consists of a network of
lymphatic vessels, nodes, and tissues where B
andT lymphocytes complete their differentiation.
Lymph nodes, which are the site where many
lymphomas originate, exhibit an outer cortex
and an inner medulla.The cortex contains
well-defined B-cell andT-cell domains.The
B-cell–dependent superficial cortex consists of
two types of follicles: immunologically inactive
follicles, called primary follicles, and active
follicles that contain germinal centers, called
secondary follicles. Most of theT cells are
contained in the paracortex, the area between the
medullary and outer superficial cortices.
