533
CHAPTER FOURTEEN
Blood
As bacteria, leukocytes, and damaged cells accumulate
in an infl
amed area, a thick fl uid called
pus
often forms and
remains while the invading microorganisms are active. If the
pus is not moved to the outside of the body or into a body
cavity, it may remain trapped in the tissues for some time.
Eventually, surrounding cells absorb it.
PRACTICE
20
How do white blood cells f
ght inFection?
21
Which white blood cells are the most active phagocytes?
22
How do white blood cells reach microorganisms outside blood
vessels?
White Blood Cell Counts
The procedure used to count white blood cells is similar
to that used for counting red blood cells. However, before
a
white blood cell count
(WBCC or WCC) is made, the red
blood cells in the blood sample are destroyed so they will
not be mistaken for white blood cells. Normally, a microliter
of blood includes 4,500 to 10,000 white blood cells.
The total number and percentages of different white
blood cell types are of clinical interest. A rise in the number
of circulating white blood cells may indicate infection. A total
number of white blood cells exceeding 10,000 per microliter
of blood constitutes
leukocytosis
(lu
ko-si-to
sis), indicating
acute infection, such as appendicitis. Leukocytosis may also
follow vigorous exercise, emotional disturbances, or great
loss of body fl
uids.
A total white blood cell count below 4,500 per microliter of
blood is called
leukopenia
(lu
ko-pe
ne-ah). Such a deF ciency
may accompany typhoid fever, infl uenza, measles, mumps,
chickenpox, AIDS, or poliomyelitis. Leukopenia may also result
from anemia or from lead, arsenic, or mercury poisoning.
A
differential white blood cell count
(DI±±) lists percent-
ages of the types of leukocytes in a blood sample. This test is
useful because the relative proportions of white blood cells
Functions of White Blood Cells
White blood cells protect against infection in various ways.
Some leukocytes phagocytize bacterial cells in the body, and
others produce antibodies.
Leukocytes can squeeze between the cells that form the
walls of the smallest blood vessels. This movement, called
diapedesis
(di
ah-pe
˘-de
sis), allows the white blood cells to
leave the circulation
(f g. 14.15)
. A series of proteins called
cellular adhesion molecules help guide leukocytes to the site
of injury. Once outside the blood, leukocytes move through
interstitial spaces using a form of self-propulsion called
ame-
boid motion.
RECONNECT
To Chapter 3, Cellular Adhesion Molecules, page 81.
The most mobile and active phagocytic leukocytes are
neutrophils and monocytes. Although neutrophils are unable
to ingest particles much larger than bacterial cells, mono-
cytes can engulf larger structures. Monocytes contain numer-
ous lysosomes, filled with digestive enzymes that break
down organic molecules in captured bacteria. Neutrophils
and monocytes often become so engorged with digestive
products and bacterial toxins that they also die.
When microorganisms invade human tissues, basophils
respond by releasing biochemicals that dilate local blood
vessels. ±or example, histamine dilates smaller blood vessels
and makes the smallest vessels leaky. As more blood fl
ows
through the smallest vessels, the tissues redden and copi-
ous fl
uids leak into the interstitial spaces. The swelling that
this inflammatory reaction produces delays the spread of
invading microorganisms into other regions (see chapter 16,
p. 627). At the same time, damaged cells release chemicals
that attract leukocytes. This phenomenon is called
positive
chemotaxis
(poz
ı˘-tiv ke
mo-tak
sis) and, when combined
with diapedesis, brings many white blood cells into infl
amed
areas quickly
(f g. 14.16)
.
Connective
tissue
Blood capillary
Leukocyte
FIGURE 14.15
In a type oF movement called diapedesis, leukocytes
squeeze between the cells oF a capillary wall and enter the tissue space
outside the blood vessel.
FIGURE 14.14
The lymphocyte, the smallest oF the white blood
cells, has a large, round nucleus (2,000
×
). Lymphocytes carry out the
immune response and are discussed Further in chapter 16.
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