is now “labeled” for phagocytosis by macrophages or
neutrophils. The antigen–antibody complex also stim-
ulates the process of complement fixation (see Box
14–3: Diagnostic Tests).
Some of the circulating complement proteins are
activated, or fixed, by an antigen–antibody complex.
Complement fixation may be complete or partial. If
the foreign antigen is cellular, the complement pro-
teins bond to the antigen–antibody complex, then to
one another, forming an enzymatic ring that punches
a hole in the cell to bring about the death of the cell.
This is complete (or entire) complement fixation and
is what happens to bacterial cells (it is also the cause of
hemolysis in a transfusion reaction).
If the foreign antigen is not a cell—let’s say it’s a
virus for example—partial complement fixation takes
place, in which some of the complement proteins bond
to the antigen–antibody complex. This is a chemotac-
tic factor. Chemotaxismeans “chemical movement” and
is actually another label that attracts macrophages to
engulf and destroy the foreign antigen.
In summary, adaptive immunity is very specific,
does create memory, and because it does, often be-
comes more efficient with repeated exposures.
Antibody Responses
The first exposure to a foreign antigen does stimulate
antibody production, but antibodies are produced
slowly and in small amounts (see Fig. 14–9). Let us
take as a specific example the measles virus. On a per-
son’s first exposure to this virus, antibody production is
usually too slow to prevent the disease itself, and the
person will have clinical measles. Most people who get
measles recover, and upon recovery have antibodies
and memory cells that are specific for the measles
virus.
On a second exposure to this virus, the memory
cells initiate rapid production of large amounts of anti-
bodies, enough to prevent a second case of measles.
This is the reason why we develop immunity to certain
diseases, and this is also the basis for the protection
given by vaccines(see Box 14–4: Vaccines).
As mentioned previously, antibodies label patho-
gens or other foreign antigens for phagocytosis or
complement fixation. More specifically, antibodies
cause agglutination or neutralization of pathogens
before their eventual destruction. Agglutination
means “clumping,” and this is what happens when
antibodies bind to bacterial cells. The bacteria that are
clumped together by attached antibodies are more eas-
ily phagocytized by macrophages (see Fig. 14–8).
The activity of viruses may be neutralized by anti-
bodies. A virus must get inside a living cell in order to
reproduce itself. However, a virus with antibodies
attached to it is unable to enter a cell, cannot repro-
duce, and will soon be phagocytized. Bacterial toxins
may also be neutralized by attached antibodies. The
antibodies change the shape of the toxin, prevent it
from exerting its harmful effects, and promote its
phagocytosis by macrophages.
Allergiesare also the result of antibody activity
(see box 14–5: Allergies).TYPES OF IMMUNITY
If we consider the source of immunity, that is, where
it comes from, we can begin with two major cate-
gories: genetic immunity and acquired immunity.
Genetic immunity is conferred by our DNA, and
acquired immunity is developed or acquired by natu-
ral or artificial means.
Genetic immunitydoes not involve antibodies or
the immune system; it is the result of our genetic
makeup. What this means is that some pathogens334 The Lymphatic System and Immunity
BOX14–3 DIAGNOSTIC TESTS
Several important laboratory tests involve anti-
bodies and may be very useful to confirm a diag-
nosis.
Complement fixation test—determines the
presence of a particular antibody in the patient’s
blood, but does not indicate when the infection
occurred.
Antibody titer—determines the level or
amount of a specific antibody in the patient’s
blood. If another titer is done 1 to several weeks
later, an increase in the antibody level shows the
infection to be current.
Fluorescent antibody test—uses antibodies
tagged with fluorescent dyes, which are added
to a clinical specimen such as blood, sputum, or
a biopsy of tissue. If the suspected pathogen is
present, the fluorescent antibodies will bond to it
and the antigen–antibody complex will “glow”
when examined with a fluorescent microscope.
Tests such as these are used in conjunction
with patient history and symptoms to arrive at a
diagnosis.