780
UNIT 4
Maintenance of the Body
21
dramatically reduced hepatitis B, tetanus, influenza, and pneu-
monia in adults, immunization of adults in the U.S. has a much
lower priority than that of children. As a result more than 65,000
Americans die each year from vaccine-preventable infections.
Conventional vaccines have shortcomings. In extremely rare
cases, vaccines have caused the very disease they are trying to
prevent because the attenuated virus wasn’t weakened enough.
In some individuals, contaminating proteins (for example, egg
albumin) cause allergic responses to the vaccine. Te new “na-
ked DNA” antiviral vaccines, blasted into the skin with a gene
gun, and
edible
vaccines taken orally appear to circumvent these
problems, but are not always effective.
Passive humoral immunity
differs from active immunity, both
in the antibody source and in the degree of protection it provides
(Figure 21.13). Instead of being made by your plasma cells, ready-
made antibodies are introduced into your body. As a result, your
B cells are not challenged by antigens, immunological memory
does not occur, and the protection provided by the “borrowed”
antibodies ends when they naturally degrade in the body.
Passive immunity is conferred
naturally
on a fetus or infant
when the mother’s antibodies cross the placenta or are ingested
with the mother’s milk. For several months a±er birth, the baby
is protected from all the antigens to which the mother has been
exposed.
Passive immunity can also be conferred
artificially
by admin-
istering exogenous antibodies (from outside your own body) as
gamma globulin
, harvested from the plasma of an immune do-
nor. Exogenous antibodies are used to prevent hepatitis A and
treat poisonous snake bites (antivenom), botulism, rabies, and
tetanus (antitoxin) because these rapidly fatal diseases would
kill a person before active immunity could be established. Te
donated antibodies provide immediate protection, but their ef-
fect is short-lived (two to three weeks).
Check Your Understanding
13.
Why is the secondary response to an antigen so much faster
than the primary response?
14.
How do vaccines protect against common childhood illnesses
such as chicken pox, measles, and mumps?
For answers, see Appendix H.
Antibodies
Describe the structure of an antibody monomer, and name
the five classes of antibodies.
Explain the function(s) of antibodies and describe clinical
uses of monoclonal antibodies.
Antibodies
, also called
immunoglobulins (Igs)
(im
0
u-no-glob
9
u-
linz), constitute the
gamma globulin
part of blood proteins. As we
mentioned earlier, antibodies are proteins secreted in response to
an antigen by effector B cells called plasma cells, and the antibod-
ies bind specifically with that antigen. Tey are formed in response
to an incredible number of different antigens.
Despite their variety, all antibodies can be grouped into one
of five Ig classes, each slightly different in structure and function.
for long periods in humans and many retain their capacity to
produce powerful secondary humoral responses for life.
Te same general events occur in the cellular immune re-
sponse: A primary response sets up a pool of effector cells (in
this case, ² cells) and generates memory cells that can then
mount secondary responses.
Active and Passive Humoral Immunity
Compare and contrast active and passive humoral
immunity.
When your B cells encounter antigens and produce antibodies
against them, you are exhibiting
active humoral immunity
. Ac-
tive immunity is acquired in two ways
(Figure 21.13)
. It is (1)
naturally acquired
when you get a bacterial or viral infection,
during which time you may develop symptoms of the disease
and suffer a little (or a lot), and (2)
artificially acquired
when
you receive
vaccines
. Indeed, once researchers realized that
secondary responses are so much more vigorous than primary
responses, the race was on to develop vaccines to “prime” the im-
mune response by providing a first encounter with the antigen.
Most vaccines contain pathogens that are dead or
attenuated
(living, but extremely weakened), or their components. Vac-
cines provide two benefits:
Tey spare us most of the symptoms and discomfort of the dis-
ease that would otherwise occur during the primary response.
Teir weakened antigens provide functional antigenic deter-
minants that are both immunogenic and reactive.
Vaccine
booster shots
, which may intensify the immune re-
sponse at later encounters with the same antigen, are also available.
Vaccines have wiped out smallpox and have substantially
lessened the illness caused by such former childhood killers as
whooping cough, polio, and measles. Although vaccines have
Passive
Active
Humoral
immunity
Artificially
acquired
Injection of
exogenous
antibodies
(gamma
globulin)
Naturally
acquired
Antibodies
passed from
mother to
fetus via
placenta; or
to infant in
her milk
Artificially
acquired
Vaccine;
dead or
attenuated
pathogens
Naturally
acquired
Infection;
contact with
pathogen
Figure 21.13
Active and passive humoral immunity.
Active
immunity establishes immunological memory; passive immunity
never does.
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