Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

A HIERARCHICAL STEM-CELL SYSTEM: BLOOD CELL FORmation
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Table 22–1 Blood Cells
Type of cell Main functions Typical concentration in
human blood (cells/liter)
Red blood cells (erythrocytes) Transport O 2 and CO 2 5 × 10 12
White blood cells (leukocytes)
Granulocytes
Neutrophils (polymorphonuclear
leukocytes)
Eosinophils
Basophils
Monocytes
Phagocytose and destroy invading bacteria 5 × 10 9
Destroy larger parasites and modulate allergic
inflammatory responses
Release histamine (and in some species serotonin) in
certain immune reactions
Become tissue macrophages, which phagocytose and
digest invading microorganisms and foreign bodies as
well as damaged senescent cells
2 × 10 8
4 × 10 7
4 × 10 8
Lymphocytes
B cells Make antibodies 2 × 10 9
T cells
Kill virus-infected cells and regulate activities of other
1 × 10 9
leukocytes
Natural killer (NK) cells Kill virus-infected cells and some tumor cells 1 × 10 8
Platelets (cell fragments arising from
megakaryocytes in bone marrow)
Initiate blood clotting 3 × 10 11
Humans contain about 5 liters of blood, accounting for 7% of body weight. Red blood cells constitute about 45% of this volume and white blood
cells about 1%, the rest being the liquid blood plasma.
blood cells, as well as by the activation of complement (discussed in Chapter 24).
Some of these signal molecules act on the endothelial lining of nearby capillaries,
helping white blood cells to first stick and then make an exit from the bloodstream
into the tissue where they are needed, as described in Chapter 19 (see Figure
19–28 and Movie 19.2). Damaged or inflamed tissues and local endothelial cells
secrete other molecules called chemokines, which act as chemoattractants for
specific types of white blood cells, causing them to become polarized and crawl
toward the source of the attractant. As a result, large numbers of white blood cells
enter the affected tissue (Figure 22–28).
Other signal molecules produced during an inflammatory response escape
into the blood and stimulate the bone marrow to produce more leukocytes and
release them into the bloodstream. The regulation tends to be cell-type specific:
some bacterial infections, for example, cause a selective increase in neutrophils,
while infections with some protozoa and other parasites cause a selective increase
in eosinophils. (For this reason, physicians routinely use differential white blood
cell counts to aid in the diagnosis of infectious and other inflammatory diseases.)
In other circumstances, erythrocyte production is selectively increased—for
example, in response to anemia (lack of hemoglobin) due to blood loss, and in the
process of acclimatization when one goes to live at high altitude, where oxygen is
scarce. Thus, blood cell formation, or hematopoiesis, necessarily involves complex
controls, which regulate the production of each type of blood cell individually to
meet changing needs.
Figure 22–28 Chemotaxis of white blood cells to damaged tissue.
A chemoattractive signal released from a site of damage, which is toward
the bottom of the page, causes white blood cells to exit from the capillary by
crawling between adjacent endothelial cells, as shown.
endothelial cell
10 µm
basal lamina
white blood cell in capillary
EXPOSURE TO MEDIATORS
OF INFLAMMATION RELEASED
FROM DAMAGED TISSUE
CHEMOTAXIS TOWARD
ATTRACTANTS RELEASED
FROM DAMAGED TISSUE
white blood cells in connective tissue