Chapter 21
The Immune System: Innate and Adaptive Body Defenses
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21
circulate in blood while others are found primarily in body se-
cretions, some cross the placental barrier, and so on.
Antibody Classes
Te five major immunoglobulin classes are designated IgM,
IgA, IgD, IgG, and IgE, on the basis of the C regions in their
heavy chains. (Remember the name MADGE to recall the five
Ig types.)
Te antibodies of each class have different characteristics, bio-
logical roles, and locations in the body, as shown in
Table 21.4
.
IgM in plasma is huge compared to the other antibodies. It is con-
structed from five Y-shaped units, or
monomers
, linked together
to form a
pentamer
(
penta
5
five). IgA occurs in both monomer
and
dimer
(two linked monomers) forms. IgD, IgG, and IgE are
monomers and have the same basic Y-shaped structure.
A single B cell can switch from making one class of antibody
to another, thereby producing two or more different antibody
classes having the same antigen specificity. For example, the first
antibody released in the primary response is IgM, and then later
plasma cells begin to secrete IgG. During secondary responses,
almost all of the Ig protein is IgG.
Antibody Targets and Functions
Tough antibodies themselves cannot destroy antigens, they can
inactivate antigens and tag them for destruction
(Figure 21.15)
.
Te common event in all antibody-antigen interactions is forma-
tion of
antigen-antibody
(or
immune
)
complexes
. Defensive
mechanisms used by antibodies include neutralization, agglutina-
tion, precipitation, and complement fixation, with the first two
most important.
Before seeing how these Ig classes differ from one another, let’s
look at how all antibodies are alike.
Basic Antibody Structure
Regardless of its class, each antibody consists of four looping
polypeptide chains linked together by disulfide (sulfur-to-sulfur)
bonds. Te four chains combined form a molecule, called an
antibody monomer
(mon
9
o-mer), with two identical halves. Te
molecule as a whole is ± or Y shaped
(Figure 21.14)
.
±wo of the chains, called the
heavy (H) chains
, are identical to
each other (blue chains in Figure 21.14a). Te other two chains,
called the
light (L) chains
(pink), are also identical to each other,
but they are only about half as long as each H chain. Te heavy
chains have a flexible
hinge
region at their approximate “middles.”
Te “loops” on each chain are created by disulfide bonds that cause
the intervening parts of the polypeptide chains to loop out.
Each chain forming an antibody has a
variable (V) region
at
one end and a
constant (C) region
at the other end. Antibodies
responding to different antigens have very different V regions,
but their C regions are the same (or nearly so) in all antibodies of
a given class. In each arm of the monomer, the V regions of the
heavy and light chains combine to form an
antigen-binding site
shaped to “fit” a specific antigenic determinant. Consequently,
each antibody monomer has two such antigen-binding regions.
Te C regions that form the
stem
of the antibody monomer
determine the antibody class and serve common functions in
all antibodies: Tese are the
effector regions
of the antibody that
dictate (1) the cells and chemicals of the body the antibody can
bind to, and (2) how the antibody class functions to eliminate
antigens. For example, some antibodies fix complement, some
Antigen-binding
site
Adaptive defenses
Humoral immunity
Heavy chain
Light chain
Stem region
Hinge region
Disulfide bond
(a)
(b)
Light chain
constant region
Light chain
variable region
Heavy chain
constant region
Heavy chain
variable region
Figure 21.14
Antibody structure.
(a)
Schematic antibody structure (based on
IgG) consists of four polypeptides—two
short
light chains
and two long
heavy chains
joined together by disulfide bonds (S–S).
Each chain has a V (variable) region (which
differs in antibodies from different cells) and
a C (constant) region (essentially identical in
different antibodies of the same class).
Together, the variable regions form the
antigen-binding sites—two per antibody
monomer.
(b)
Computer-generated image of
antibody structure.
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