Chapter 21
The Immune System: Innate and Adaptive Body Defenses
783
21
Table 21.4
Immunoglobulin Classes*
IgM (pentamer)
The first immunoglobulin class se-
creted by plasma cells during the
primary response.
(This fact is diag-
nostically useful because presence
of IgM in plasma usually indicates
current infection by the pathogen
eliciting IgM’s formation.)
Readily fixes and activates
complement.
Exists in monomer and pentamer
(five united monomers) forms.
The monomer serves as an antigen
receptor on the B cell surface.
The pentamer (illustrated) circulates
in blood plasma.
Numerous antigen-binding sites
make it a potent agglutinating agent.
IgA (dimer)
The dimer (illustrated), referred to
as
secretory IgA
, is found in body
secretions such as saliva, sweat, in-
testinal juice, and milk.
Secretory IgA helps stop pathogens
from attaching to epithelial cell sur-
faces (including mucous membranes
and the epidermis).
The monomer exists in limited
amounts in plasma.
IgD (monomer)
Found on the B cell surface.
Functions as a B cell antigen recep-
tor (as does IgM).
IgG (monomer)
The most abundant antibody in
plasma, accounting for 75–85% of
circulating antibodies.
The main antibody of both second-
ary and late primary responses.
Readily fixes and activates
complement.
Protects against bacteria, viruses, and
toxins circulating in blood and lymph.
Crosses the placenta and confers
passive immunity from the mother
to the fetus.
IgE (monomer)
Stem end binds to mast cells or
basophils. Antigen binding to its
receptor end triggers these cells to
release histamine and other chemi-
cals that mediate inflammation and
an allergic reaction.
Secreted by plasma cells in skin,
mucosae of the gastrointestinal and
respiratory tracts, and tonsils.
Only traces of IgE are found in
plasma.
Levels rise during severe allergic at-
tacks or chronic parasitic infections
of the gastrointestinal tract.
*Key characteristics are listed in blue type.
Homeostatic Imbalance
21.2
Around the world, billions of people are infected by parasitic
worms such as
Ascaris
and
Schistosoma
. Tese large pathogens
are difficult for our immune systems to deal with and “PLAN”
is insufficient.
Nevertheless, antibodies still play a critical role in the worm’s
destruction. IgE antibodies coat the surface of parasitic worms,
marking them for destruction by eosinophils. When eosinophils
encounter antibody-coated worms, they bind to the exposed
stems of the IgE. Tis triggers the eosinophils to release the toxic
contents of their large cytoplasmic granules all over their prey.
Monoclonal Antibodies as Clinical
and Research Tools
In addition to their role in providing passive immunity, com-
mercially prepared antibodies are essential in research, clinical
testing, and treatment.
Monoclonal antibodies
, produced by
descendants of a single cell, are pure antibody preparations spe-
cific for a single antigenic determinant.
Monoclonal antibodies are made by fusing tumor cells and
B lymphocytes. Te resulting cell hybrids, called
hybridomas
(hi
0
brĭ-do
9
mahz), have desirable traits of both parent cells. Like
tumor cells, hybridomas proliferate indefinitely in culture, and
like B cells, they produce a single type of antibody.
Monoclonal antibodies are used to diagnose pregnancy,
certain sexually transmitted diseases, some cancers, hepatitis,
and rabies. Tese monoclonal antibody tests are more specific,
sensitive, and rapid than other tests. Monoclonal antibodies are
also used to treat leukemia and lymphomas, cancers that are
present in the circulation and so are easily accessible to injected
antibodies. Tey also serve as “guided missiles” to deliver anti-
cancer drugs only to cancerous tissue, and to treat certain au-
toimmune diseases (as we will discuss later).
Summary of Antibody Actions
At the most basic level, the race between antibody production
and pathogen multiplication determines whether or not you be-
come sick. Remember, however, that forming antigen-antibody
complexes does
not
destroy the antigens. Instead, it prepares
them for destruction by innate defenses.
Antibodies produced by plasma cells are in many ways the
simplest, most versatile ammunition of the immune response.
Nevertheless, they provide only partial immunity. Teir prey are
extracellular pathogens
—intact bacteria, free viruses, and soluble
foreign molecules—in other words, pathogens that are free in body
secretions and tissue fluid and circulating in blood and lymph.
Antibodies never invade solid tissues unless a lesion is present.
Until recently, the accepted dogma was that antibodies
only
act extracellularly. Remarkably, we now know that antibodies
can act intracellularly as well. Antibodies that are attached to a
virus before that virus infects a cell can “hang on” to the virus as
it slips inside the cell. Tere, the antibodies activate intracellular
mechanisms that destroy the virus. Even so, antibodies are not
very effective against pathogens like viruses and tuberculosis
bacilli that quickly slip inside body cells to multiply there. For
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