If pathogens
penetrate
the
barriers that make up the first
line of defense, several internal
mechanisms serve as a second line
of defense. These mechanisms in-
clude (1) antimicrobial chemicals,
(2)
phagocytosis
(see
Chapter
3), (3) inflammation, and (4) fe-
ver.
Figure 12.7
explains all four
mechanisms in greater depth.
There are four types of anti-
microbial substances (Figure 12.7a). Complement pro-
teins from blood form holes in microbial membranes,
which causes them to burst. Interferons are proteins
produced by lymphocytes, macrophages, and fibroblasts
that migrate to infected cells and interfere with viral rep-
lication. Iron-binding proteins inhibit the growth of
certain bacteria by reducing the amount of available iron.
Antimicrobial proteins are short peptides that kill mi-
crobes and attract other cells that participate in immune
responses. When microbes penetrate the skin and mucous
membranes or bypass the antimicrobial defenses in the
blood, the next nonspecific defense consists of phagocytes
and T cells and B cells (Figure 12.7b).
Inflammation (Figure 12.7c) is a defensive response
of the body to tissue damage. Because inflammation is
one of the body’s innate defenses, the response of a tissue
to a cut is similar to damage caused by burns, radiation,
phagocytosis
(fag'-o-sT-TO-sis) The
process by which
phagocytes ingest
particulate matter; the
ingestion and destruc-
tion of microbes, cell
debris, and other
foreign matter.
The seco n d line o f in n ate im m unity • Figure 12.7
Various mechanisms, including chemicals, killer cells, inflammation, and fever,
make up a second line of defense when microbes get past surface barriers.
a.
Antimicrobial substances
Complement
Complement proteins from blood become
activated and form holes in microbial
membranes, thereby causing them to burst
(cytolysis). Complement also
^ C3
attracts phagocytes
to a site (chemotaxis).
C5
C5b
C5a
PHAGOCYTOSIS:
Enhancement of phagocytosis v____
J
by coating with C3b
____
ΠX
V
ΠD
ŒD
C6
C5b Channel
Membrane attack
complex forms
channel
Microbial
plasma
membrane
CYTOLYSIS:
Bursting of microbe due to inflow of
extracellular fluid through channel formed
by membrane attack complex C5-C9
Interferons
Lymphocytes, macrophages, and fibroblasts infected with viruses
produce proteins called interferons that interfere with viral
replication. Interferons diffuse from one cell to another, rendering
the receiving cells resistant to the viruses.
Antimicrobial proteins
Antimicrobial proteins (AMPs) are short peptides that
kill microbes and attract mast cells and dendritic
cells, which participate in immune responses.
Microbes do not develop resistance to AMPs as they
often do to antibiotics.
Iron-binding proteins
Iron-binding proteins reduce
available iron and inhibit the
growth of microbes.
These proteins include:
Hem oglobin
in red blood
cells
Transferrin
in blood and
tissue fluids
Ferretin
in the liver,
spleen, and red bone
marrow
Lactoferrin
in milk, saliva,
and mucus
Beta polypeptide
chains (globins)
Heme
Alpha polypeptide
chains (globins)
Iron (Fe)
Hemoglobin molecule
352 CHAPTER 12
The Lymphatic System and Immunity
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