136
UNIT 2
Basic Tissues
host cells, and
helper T cells
and
suppressor T cells
regulate
immune responses (Fig. 8-4 A,C). For null cells, see Chapter
10, “Lymphoid System.”
Monocytes
,
the largest of the leukocytes, constitute about
3% to 7% of circulating white blood cells. Monocytes are not
functionally or structurally mature when they are
circulating in
blood. Rather, they are an intermediate form of a cell lineage
that starts differentiation in the bone marrow (like all blood
cells) but does not fully complete differentiation until arrival in a
peripheral tissue. There are several functional cell types that are
derived from monocytes, including
tissue macrophages
,
Kupffer
cells
,
microglial
cells
,
osteoclasts
, and
antigen-presenting
cells
(Fig. 8-4B).
Neutrophils
are by far the most abundant leukocytes. Typi-
cally 54% to 62% of leukocytes are mature neutrophils, and an
additional 3% to 5% of leukocytes are immature (band) forms.
Neutrophils are the main cellular weapon for destroying bac-
teria, and the total number of circulating neutrophils can rise
sharply in response to bacterial infections. Once a neutrophil
makes contact with a bacterium, the neutrophil attaches to the
bacterium and engulfs it (
phagocytosis
) within a phagosome.
The primary and secondary granules of the neutrophil fuse with
the phagosome, thereby exposing the bacterium to an array
of bactericidal compounds and enzymes. Another bacteria-
killing mechanism employed by neutrophils is the generation of
reactive oxygen compounds in a process termed the
respiratory
burst
(Figs. 8-5 to 8-6).
Eosinophils
amount to about 1% to 3% of
circulating leuko-
cytes, and, like basophils, their numbers tend to rise in response
to parasitic infections and allergic reactions. The functions of
eosinophils are not fully understood, but they clearly function
in defense against infection by parasitic worms such as schisto-
somes. Eosinophils are recruited to sites of parasitic infection,
and some of their granule contents (e.g., major basic proteins)
are highly toxic to parasites. They also appear to
function in
dampening and limiting infl
ammation at sites of allergic reac-
tions (Fig. 8-7A).
Basophils
are the least numerous (<1%) of the leukocytes.
Basophils are functionally similar to the mast cells found in
connective tissue. Neither cell type functions by phagocytosis,
so they are quite unlike neutrophils. Instead, their functions in
defense against microbial invasion are indirect. When activated,
they secrete (or
exocytose
) a variety of infl
ammatory media-
tors from their granules and synthesize and release a number of
arachidonic acid derivatives, such as
leukotrienes
and
prosta-
glandins
. These signaling molecules intensify infl ammation by
(1) increasing local blood fl
ow, (2) enhancing leakage of plasma
proteins from blood, (3) promoting recruitment of other leuko-
cytes to a site of infection, and (4) enhancing the activity of the
other leukocytes (Fig. 8-7).
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