156 Chapter 9adaptive immunity Set of
immune defenses that are
tailored to the particular
pathogens that enter the
body.
antigen Molecule or parti-
cle that the immune system
recognizes as nonself.
B cell B lymphocyte. Cells
derived from them make
antibodies.
basophil Circulating
white blood cell; factor in
inflammation.
cytokines Signaling chemi-
cals released by cells of the
immune system.
dendritic cell Phagocytic
white blood cell; dendritic
cells can mobilize adaptive
immunity.
eosinophil White blood cell
that targets large parasites,
such as worms.
immune system System of
interacting white blood cells
that defend the body.
immunity The body’s ability
to resist and fight infections.
innate immunity The
body’s inborn, general
defenses against infection.
lymphocytes B cells, T
cells, and other white blood
cells that are active mainly
in tissues and organs of the
lymphatic system.
macrophage Phagocytic
white blood cell in tissue
fluid.
mast cell White blood cell
in many tissues; role in
inflammation.
neutrophil Phagocyte that
follows chemical trails to
infected, inflamed, or dam-
aged tissues.
T cell T lymphocyte. T cells
target abnormal body cells,
among other roles in adap-
tive immunity.
overview of Body Defenses
proteins, lipids, or the large sugar molecules called oligo-
saccharides. Immunity is the body’s overall ability to resist
and combat something that is nonself.We are born with some general immune
defenses and acquire other, specific ones
There are two categories of immune responses. Each of us
is born with some preset responses to infection that are car-
ried out by certain white blood cells and proteins in blood.
These activities are the body’s second line of defense.
They are triggered by chemical cues (such as certain
proteins) that are present on or in a variety of pathogens.
The responses begin within minutes and they provide
innate immunity.
Innate immune responses are general, rather like a dis-
infectant that can kill many different species of bacteria in
a bathroom. Even so, innate responses can wipe out many
invaders before an infection sets in.
When an innate immune response starts, it also
unleashes the third line of defense, or adaptive immunity.
Our adaptive immunity changes as we go through life. Its
responses are tailored to target the particular attackers
that chance to enter the body—a given species of bacteria,
a particular virus, a toxin, or an abnormal cell such as a
cancer cell.
In adaptive immunity, huge numbers of white blood
cells mount a counterattack against the invasion. The cells
in this “army” all have receptors for a particular antigen
and destroy anything that bears it. Biologists estimate that
adaptive immunity can produce white blood cell armies
with receptors for billions of different antigens. This means
that adaptive responses can be mounted against a vast
array of potential pathogens. Adaptive immune responses
take a week or so to develop, but they leave behind cells
that “remember” an antigen and protect against it for a
long time, perhaps even for life. Table 9.1 compares basic
aspects of innate and adaptive immunity.n body defenses include physical barriers and two interacting
sets of cells and proteins.
n Links to skin 4.9, blood cells 8.1three lines of defense
protect the body
We can’t really avoid the viruses,
bacteria, fungi, protozoa, and para-
sitic worms that cause disease. These
pathogens are in the air we breathe,
the food we eat, and on everything
we touch. This means that our
survival depends on having effec-
tive defenses against them. Biolo-
gists sometimes portray the body’s
anti-infection mechanisms as three
“lines of defense.” This approach
can make it easier to remember what
each “line” does, but it is important
to remember that all the defenses we
will discuss in this chapter function
as parts of a whole.
The first defensive line is the
body’s array of physical and chemi-
cal barriers to infection, such as
intact skin and the linings of body
cavities and tubes. These barriers,
which we discuss further in Section
9.3, are not part of the immune sys-
tem. Instead, the immune system
is a “cellular system” because white
blood cells perform most of its core
functions (Figure 9.1). As you’ll soon
see, the immune system’s two inter-
acting arms respond differently to
threats. One is a bit like a commu-
nity’s “first responder” squad that
rushes to the scene of a mishap.
In addition to providing immediate
“first aid,” this rapid response mobi-
lizes reinforcements—appropriate
“specialists” that can deal with par-
ticular types of threats.
You may remember from Section
8.4 that an antigen is something that
the body identifies as nonself and
that triggers an immune response.
Virus particles, foreign cells, toxins,
and cancer cells all have antigens
on their surface. Most antigens areInnate Immunity Adaptive Immunity
Response time Immediate 7–10 days
Antigen About 1,000 preset Vast number of
detection receptors receptors for
specific antigens
Triggers Nonself chemical Antigens of pathogens,
cues on or in toxins, proteins on
pathogens altered body cells
Memory None Long-termTable 9.1 innate and Adaptive immunity Compared9.1
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