Chapter 21
The Immune System: Innate and Adaptive Body Defenses
773
21
At first glance, the adaptive system seems to have a major
shortcoming. Unlike the innate system, which is always ready
and able to react, the adaptive system must “meet” or be primed
by an initial exposure to a specific foreign substance (antigen).
Only then can it protect the body against that substance, and
this priming takes precious time.
Experiments in the late 1800s revealed the basis of this specific
immunity. Researchers demonstrated that animals surviving a se-
rious bacterial infection have protective factors (the proteins we
now call
antibodies
) in their blood that defend against future at-
tacks by the same pathogen. Furthermore, researchers found that
if antibody-containing serum from the surviving animals was in-
jected into animals that had not been exposed to the pathogen,
the injected animals would also be protected. Tese landmark
experiments were exciting because they revealed three important
aspects of the adaptive immune response:
It is specific.
It recognizes and targets
particular
pathogens or
foreign substances that initiate the immune response.
It is systemic.
Immunity is not restricted to the initial infec-
tion site.
It has “memory.”
A±er an initial exposure, it recognizes and
mounts even stronger attacks on previously encountered
pathogens.
At first antibodies were thought to be the sole artillery of the
adaptive immune system. Ten, in the mid-1900s researchers dis-
covered that injecting antibody-containing serum did
not
always
protect the recipient from diseases the serum donor had survived.
In such cases, however, injecting the donor’s lymphocytes
did
pro-
vide immunity. As the pieces fell into place, researchers recognized
two separate but overlapping arms of adaptive immunity, each us-
ing a variety of attack mechanisms that vary with the intruder.
Humoral immunity
(hu
9
mor-ul), also called
antibody-
mediated immunity
, is provided by antibodies present in the body’s
“humors,” or fluids (blood, lymph, etc.). Tough they are produced
by lymphocytes, antibodies circulate freely in the blood and lymph,
where they bind primarily to
extracellular
targets—bacteria, bac-
terial toxins, and free viruses—inactivating them temporarily and
marking them for destruction by phagocytes or complement.
When lymphocytes themselves rather than antibodies defend
the body, the immunity is called
cellular
or
cell-mediated immu-
nity
because living cells provide the protection. Cellular immunity
also has
cellular
targets—virus-infected or parasite-infected tissue
cells, cancer cells, and cells of foreign gra±s. Te lymphocytes act
against such targets either
directly
, by killing the infected cells, or
indirectly
, by releasing chemicals that enhance the inflammatory
response or activate other lymphocytes or macrophages.
Before we describe the humoral and cellular responses, let’s
first consider the
antigens
that trigger the activity of the remark-
able cells involved in adaptive immunity.
Antigens
Define antigen and describe how antigens affect the
adaptive defenses.
Define complete antigen, hapten, and antigenic determinant.
membrane. MAC forms and stabilizes a hole in the membrane
that allows a massive influx of water, lysing the target cell.
Te C3b molecules that coat the microorganism provide
“handles” that receptors on macrophages and neutrophils can
adhere to, allowing them to engulf the particle more rapidly. As
noted earlier, this process is called
opsonization
. C3a and other
cleavage products formed during complement fixation amplify
the inflammatory response by stimulating mast cells and ba-
sophils to release histamine and by attracting neutrophils and
other inflammatory cells to the area.
Fever
Explain how fever helps protect the body.
Inflammation is a localized response to infection, but some-
times the body’s response to the invasion of microorganisms is
more widespread.
Fever
, or abnormally high body temperature,
is a systemic response to invading microorganisms.
Fever is an adaptive response that seems to benefit the body.
When leukocytes and macrophages are exposed to foreign sub-
stances in the body, they release chemicals called
pyrogens
(
pyro
5
fire). Tese pyrogens act on the body’s thermostat—
a cluster of neurons in the hypothalamus—raising the body’s
temperature above normal [37°C (98.6°F)].
Fever causes the liver and spleen to sequester iron and zinc,
making them less available to support bacterial growth. Addi-
tionally, fever increases the metabolic rate of tissue cells in gen-
eral, speeding up repair processes.
Check Your Understanding
3.
What is opsonization and how does it help phagocytes? Give
an example of a molecule that acts as an opsonin.
4.
Under what circumstances might NK cells kill our own cells?
5.
What are the cardinal signs of inflammation and what causes
them?
For answers, see Appendix H.
PART 2
Adaptive Defenses
Most of us would find it wonderfully convenient if we could
walk into a single clothing store and buy a complete wardrobe—
hat to shoes—that fits perfectly regardless of any special figure
problems. We know that such a service would be next to impos-
sible to find. And yet, we take for granted our
adaptive immune
system
, the body’s built-in
specific defensive system
that stalks
and eliminates with nearly equal precision almost any type of
pathogen that intrudes into the body.
When it operates effectively, the adaptive immune system
protects us from a wide variety of infectious agents, as well as
from abnormal body cells. When it fails, or is disabled, dev-
astating diseases such as cancer and AIDS result. Te activity
of the adaptive immune system tremendously amplifies the in-
flammatory response and is responsible for most complement
activation.
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