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
from the Greek word that means “to prepare
As the phagocyte engulfs the microbe (
the internal, microbe-containing sac formed in this step is
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
the phagolysosome, the lysosomal enzymes break down the
microbe’s macromolecules. In addition, other enzymes in the
phagolysosome membrane produce
as well as
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 (
) 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.
The family of plasma proteins known as
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
Macrophages contacting bacteria and preparing to engulf them.
into or out of
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.