cause disease in certain host species but not in others.
Dogs and cats, for example, have genetic immunity
to the measles virus, which is a pathogen only for peo-
ple. Mouse leukemia viruses affect only mice, not
people; we have genetic immunity to them. This is
not because we have antibodies against these mouse
viruses, but rather that we have genes that are the
codes for proteins that make it impossible for such
pathogens to reproduce in our cells and tissues.
Monkeys have similar protective genes and proteins
for the human AIDS virus; HIV does not cause disease
in these monkeys. Because this is a genetic character-
istic programmed in DNA, genetic immunity always
lasts a lifetime.
Acquired immunity does involve antibodies.
Passive immunitymeans that the antibodies are from
another source, whereas active immunitymeans that
the individual produces his or her own antibodies.
One type of naturally acquired passive immunity is
the placental transmission of antibodies (IgG) from
maternal blood to fetal circulation. The baby will then
be born temporarily immune to the diseases the
mother is immune to. Such passive immunity may be
prolonged by breast-feeding, because breast milk also
contains maternal antibodies (IgA).
Artificially acquired passive immunity is obtained
by the injection of immune globulins (gamma globu-
lins or preformed antibodies) after presumed exposure
to a particular pathogen. Such immune globulins are
available for German measles, hepatitis A and B,
tetanus and botulism (anti-toxins), and rabies. These
are notvaccines; they do not stimulate immune mech-
anisms, but rather provide immediate antibody pro-
tection. Passive immunity is always temporary, lasting
a few weeks to a few months, because antibodies from
another source eventually break down.
Active immunity is the production of one’s own
antibodies and may be stimulated by natural or artifi-
cial means. Naturally acquired active immunity means
that a person has recovered from a disease and now
has antibodies and memory cells specific for that
pathogen. Artificially acquired active immunity is the
result of a vaccine that has stimulated production of
antibodies and memory cells (see Box 14–6: Vaccines
That Have Changed Our Lives). No general state-
ment can be made about the duration of active immu-
nity. Recovering from plague, for example, confers
lifelong immunity, but the plague vaccine does not.
Duration of active immunity, therefore, varies with
the particular disease or vaccine.
The types of immunity are summarized in Table
14–2.336 The Lymphatic System and Immunity
BOX14–5 ALLERGIES
In an allergic reaction, the effects of inflamma-
tory chemicals create symptoms such as watery
eyes and runny nose (hay fever) or the more serious
wheezing and difficult breathing that characterize
asthma. Several medications are available to coun-
teract these effects (see Chapter 15 for a description
of asthma).
Anaphylactic shock is an extreme allergic
response that may be elicited by exposure to peni-
cillin or insect venoms. On the first exposure, the
person becomes highly sensitized to the foreign
antigen. On the second exposure, histamine is
released from mast cells throughout the body and
causes a drastic decrease in blood volume. The
resulting drop in blood pressure may be fatal in
only a few minutes. People who know they are
allergic to bee stings, for example, may obtain a
self-contained syringe of epinephrine to carry with
them. Epinephrine can delay the progression of
anaphylactic shock long enough for the person to
seek medical attention.Anallergyis a hypersensitivity to a particular for-
eign antigen, called an allergen. Allergens include
plant pollens, foods, chemicals in cosmetics, antibi-
otics such as penicillin, dust, and mold spores. Such
allergens are not themselves harmful. Most people,
for example, can inhale pollen, eat peanuts, or take
penicillin with no ill effects.
Hypersensitivity means that the immune system
overresponds to the allergen, and produces tissue
damage by doing so. Allergic responses are charac-
terized by the production of IgE antibodies, which
bond to mast cells. Mast cells are specialized con-
nective tissue cells and are numerous in the con-
nective tissue of the skin and mucous membranes.
Chemicals in mast cells include histamine and
leukotrienes, which are released by the bonding of
IgE antibodies or when tissue damage occurs.
These chemicals contribute to the process of
inflammation by increasing the permeability of cap-
illaries and venules. Tissue fluid collects and more
WBCs are brought to the damaged area.