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Chapter 18
carbohydrates or lipids in the microbial cell walls. Contact
is not always suffi cient to trigger engulfment, however, par-
ticularly with bacteria that are surrounded by a thick, gelati-
nous capsule. Instead, chemical factors produced by the body
can bind the phagocyte tightly to the microbe and thereby
enhance phagocytosis. Any substance that does this is known
as an
opsonin,
from the Greek word that means “to prepare
for eating.”
As the phagocyte engulfs the microbe (
Figure 18–2
),
the internal, microbe-containing sac formed in this step is
called a
phagosome.
A layer of plasma membrane separates the
microbe from the cytosol of the phagocyte. The phagosome
membrane then makes contact with one of the phagocyte’s
lysosomes, which is fi lled with a variety of hydrolytic enzymes.
The membranes of the phagosome and lysosome fuse, and the
combined vesicles are now called a
phagolysosome.
Inside
the phagolysosome, the lysosomal enzymes break down the
microbe’s macromolecules. In addition, other enzymes in the
phagolysosome membrane produce
nitric oxide
as well as
hydrogen peroxide
and other oxygen derivatives, all of which
are extremely destructive to the microbe’s macromolecules.
Such intracellular destruction is not the only way phago-
cytes can kill microbes. The phagocytes also release antimi-
crobial substances into the extracellular fl
uid, where these
chemicals can destroy the microbes without prior phagocytosis.
Some of these substances (for examp
le, nitric oxide)
secreted into the extracellular fl uid (
Figure 18–3
) also func-
tion as infl ammatory mediators. Thus, when phagocytes enter
the area and encounter microbes, positive feedback mecha-
nisms cause infl
ammatory mediators, including chemokines,
to be released that bring in more phagocytes.
Complement
The family of plasma proteins known as
complement
provides
another means for extracellular killing of microbes without
prior phagocytosis. Certain complement proteins are always
circulating in the blood in an inactive state. Upon activation
of a complement protein in response to infection or damage,
a cascade occurs so that this active protein activates a second
complement protein, which activates a third, and so on. In this
Figure 18–1
Macrophages contacting bacteria and preparing to engulf them.
Macrophages
Pseudopods
Bacteria
Microbe
(in extracellular
fluid)
Endocytosis
Phagosome
formation
Lysosome
Phagocyte
Phagolysosome
Release of
end-products
into or out of
cell
Nucleus
Figure 18–2
Phagocytosis and intracellular destruction of a microbe. After destruction has taken place in the phagolysosome, the end-products are released
to the outside of the cell by exocytosis or used by the cell for its own metabolism.
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