Sometimes a person who has been exposed to infec-
tion needs protection against a pathogen but lacks time to
develop active immunity from a vaccine. This happens with
hepatitis A, a viral infection of the liver. In such a case, it
may be possible to inject the person with antiserum, which
has ready-made antibodies from gamma globulin separated
from the blood plasma of persons who have already devel-
oped immunity against the disease.
An injection of antibodies or antitoxin (antibodies
against a toxin) provides
cially acquired passive immu-
It is called passive because the recipient’s cells do not
produce the antibodies. Such immunity is short-term, sel-
dom lasting more than a few weeks. Furthermore, because
the recipient’s lymphocytes might not have time to react to
the pathogens for which protection was needed, susceptibil-
ity to infection may persist.
During pregnancy, certain antibodies (IgG) pass from
the maternal blood into the fetal bloodstream. Receptor-
mediated endocytosis (see chapter 3, p. 96) using receptor
sites on cells of the fetal yolk sac accomplishes the transfer.
These receptor sites bind to a region common to the structure
of IgG molecules. After entering the fetal cells, the antibod-
ies are secreted into the fetal blood. The fetus acquires lim-
ited immunity against the pathogens for which the pregnant
woman has developed active immunities. Thus, the fetus has
naturally acquired passive immunity,
which may persist for
six months to a year after birth. The newborn may naturally
acquire passive immunity through breast milk as well.
summarizes the types of immunity.
Distinguish between a primary and a secondary immune response.
Explain the dif
erence between active and passive immunities.
Practical Classif
cation oF Immunity
Before vaccines against “childhood diseases” began to be
developed in the 1960s, suffering through measles, mumps,
rubella, and chickenpox was part of attending elementary
school. However, each child usually had each illness only
once, thanks to
naturally acquired active immunity.
This form
of immunity develops after a primary immune response and is
a response to exposure to a live pathogen.
Today, most children in developed countries do not
contract measles, mumps, rubella, or chickenpox because
they develop another type of active immunity, produced
in response to receiving a
¯n). A vaccine is
a preparation that includes an antigen that can stimulate a
primary immune response against a particular pathogen but
does not produce symptoms of that disease.
A vaccine might include bacteria or viruses that have
been killed or attenuated (weakened) so that they cannot
cause a serious infection, or a toxoid, a toxin from an infec-
tious organism that has been chemically altered to destroy
its dangerous effects. A “subunit” vaccine consists of a single
glycoprotein or similar large molecule from the pathogen’s
surface, which provides enough of a foreign antigen to alert
the immune system. A vaccine causes a person to develop
cially acquired active immunity.
Although an individual receives a vaccine, the ultimate
effect is at the population level. That is, if a critical number
of people are vaccinated, becoming immune, the infectious
agent can no longer easily pass from person to person. This
protection that results from widespread vaccination is called
herd immunity.
Viruses whose genetic material rapidly mutates pre-
sent a great challenge to vaccine development because their
surfaces, which serve as antigens, change. It is a little like
fighting an enemy who is constantly changing disguises.
For this reason, pharmaceutical companies must develop a
new vaccine against infl
uenza each year. HIV is particularly
changeable, which has severely hampered efforts to produce
a vaccine.
Plasma antibody concentration
Primary response
Secondary response
Days after exposure to antigen
FIGURE 16.21
A primary immune response causes less-vigorous
antibody production than does a secondary immune response.
Vaccines stimulate active immunity against a variety oF diseases,
including typhoid Fever, cholera, whooping cough, diphtheria, teta-
nus, polio, inFluenza, hepatitis A and B, and bacterial pneumonia.
Vaccines have virtually eliminated natural smallpox From the world.
Vaccine distribution is not equitable worldwide. Many thousands oF
people in underdeveloped countries die oF inFectious diseases For
which vaccines are available in other nations.
±or some inFections, science makes developing a vaccine di²
This is the case For norovirus inFection, also known as “winter vomit-
ing disease” or erroneously as “stomach ³
u” (in³
uenza is respiratory).
The virus and receptors For it on small intestine lining cells are diverse,
so the virus always ´
nds a place to inFect. Norovirus passes readily in
Feces and vomit, especially in crowded areas such as cruise ships and
schools. Some people do not develop symptoms, yet can pass the
virus, and immunity does not last until the next winter. Practically, a
vaccine is not a priority because For most people, the illness resolves
quickly and is not severe.
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