642
UNIT 4
Maintenance of the Body
17
(immunoglobulins) that are released to the blood. (We describe
B and T lymphocyte functions in Chapter 21.)
Monocytes
Monocytes
account for 3–8% of WBCs. With an
average diameter of 18 μm, they are the largest leukocytes. ±ey
have abundant pale-blue cytoplasm and a darkly staining purple
nucleus, which is distinctively U or kidney shaped (Table 17.2
and Figure 17.10e).
When circulating monocytes leave the bloodstream and
enter the tissues, they differentiate into highly mobile
macro-
phages
with prodigious appetites. Macrophages are actively
phagocytic, and they are crucial in the body’s defense against
viruses, certain intracellular bacterial parasites, and
chronic
in-
fections such as tuberculosis. As we explain in Chapter 21, mac-
rophages are also important in activating lymphocytes to mount
the immune response.
Production and Life Span of Leukocytes
Like erythropoiesis,
leukopoiesis
, or the production of white
blood cells, is stimulated by chemical messengers. ±ese mes-
sengers, which can act either as paracrines or hormones, are
glycoproteins that fall into two families of hematopoietic fac-
tors,
interleukins
and
colony-stimulating factors
, or
CSFs
.
±e interleukins are numbered (e.g., IL-3, IL-5), but most CSFs
are named for the leukocyte population they stimulate—for ex-
ample,
granulocyte-CSF
(
G-CSF
) stimulates production of gran-
ulocytes. Hematopoietic factors, released by supporting cells of
the red bone marrow and mature WBCs, not only prompt the
white blood cell precursors to divide and mature, but also en-
hance the protective potency of mature leukocytes.
Homeostatic Imbalance
17.2
Many of the hematopoietic hormones (EPO and several of the
CSFs) are used clinically. ±ese hormones stimulate the bone
marrow of cancer patients who are receiving chemotherapy
(which suppresses the marrow) and of those who have received
stem cell transplants, and to beef up the protective responses of
AIDS patients.
Figure 17.11
shows the pathways of leukocyte differentia-
tion, starting with the hematopoietic stem cell that gives rise to
all of the formed elements in the blood. An early branching of
the pathway divides the
lymphoid stem cells
, which produce
lymphocytes, from the
myeloid stem cells
, which give rise to
all other formed elements. In each granulocyte line, the com-
mitted cells, called
myeloblasts
(mi
9
ĕ-lo-blasts
0
), accumulate
lysosomes, becoming
promyelocytes
. ±e distinctive granules
of each granulocyte type appear next in the
myelocyte
stage and
then cell division stops. In the subsequent stage, the nuclei arc,
producing the
band cell
stage. Just before granulocytes leave the
marrow and enter the circulation, their nuclei constrict, begin-
ning the process of nuclear segmentation.
±e bone marrow stores mature granulocytes and usually con-
tains about ten times more granulocytes than are found in the
blood. ±e normal ratio of granulocytes to erythrocytes produced
is about 3:1, which reflects granulocytes’ much shorter life span
(0.25 to 9.0 days). Most die combating invading microorganisms.
enzymes. However, unlike typical lysosomes, they lack enzymes
that specifically digest bacteria.
±e most important role of eosinophils is to lead the counter-
attack against parasitic worms, such as flatworms (tapeworms
and flukes) and roundworms (pinworms and hookworms) that
are too large to be phagocytized. ±ese worms are ingested in
food (especially raw fish) or invade the body via the skin and
then typically burrow into the intestinal or respiratory mucosae.
Eosinophils reside in the loose connective tissues at the same
body sites, and when they encounter a parasitic worm “prey,”
they gather around and release the enzymes from their cyto-
plasmic granules onto the parasite’s surface, digesting it away.
Eosinophils have complex roles in many other diseases in-
cluding allergies and asthma. While they contribute to the tis-
sue damage that occurs in many immune processes, we are also
beginning to recognize them as important modulators of the
immune response.
Basophils
Basophils
are the rarest white blood cells, account-
ing for only 0.5–1% of the leukocyte population. ±eir cytoplasm
contains large, coarse, histamine-containing granules that have
an affinity for the basic dyes (
basophil
5
base loving) and stain
purplish-black (Figure 17.10c).
Histamine
is an inflammatory
chemical that acts as a vasodilator (makes blood vessels dilate) and
attracts other white blood cells to the inflamed site; drugs called
antihistamines counter this effect. ±e deep purple nucleus is gen-
erally U or S shaped with one or two conspicuous constrictions.
Granulated cells similar to basophils, called
mast cells
, are
found in connective tissues. Although mast cell nuclei tend to
be more oval than lobed, the cells are similar microscopically,
and both cell types bind to a particular antibody (immunoglob-
ulin E) that causes the cells to release histamine. However, they
arise from different cell lines.
Agranulocytes
±e
agranulocytes
include lymphocytes and monocytes, WBCs
that lack
visible
cytoplasmic granules. Although similar to each
other structurally, they are functionally distinct and unrelated
cell types. ±eir nuclei are typically spherical or kidney shaped.
Lymphocytes
Lymphocytes
, accounting for 25% or more of
the WBC population, are the second most numerous leukocytes
in the blood. When stained, a typical lymphocyte has a large,
dark-purple nucleus that occupies most of the cell volume. ±e
nucleus is usually spherical but may be slightly indented, and it is
surrounded by a thin rim of pale-blue cytoplasm (Table 17.2 and
Figure 17.10d). Lymphocyte diameter ranges from 5 to 17 μm,
but they are o²en classified according to size as small (5–8 μm),
medium (10–12 μm), and large (14–17 μm).
Large numbers of lymphocytes exist in the body, but rela-
tively few (mostly the small lymphocytes) are found in the
bloodstream. In fact, lymphocytes are so called because most are
closely associated with lymphoid tissues (lymph nodes, spleen,
etc.), where they play a crucial role in immunity.
T lymphocytes
(T cells)
function in the immune response by acting directly
against virus-infected cells and tumor cells.
B lymphocytes
(B cells)
give rise to
plasma cells
, which produce
antibodies
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