section 35.6
Immunoglobulin Structure and Function
817
TABLE 35-2
Immunoglobulin Classes
IgG
IgM
IgA
IgE
IgD
Molecular weight
150,000
950,000
160,000
190,000
180,000
Molecular form1
Monomer (1)
Pentamer (5)
Monomer (1),
dimer
Monomer (1)
Monomer (1)
Sedimentation Coeffecient2
7S
19S
7S (monomer),
11S (dimer)
8S
7S
Heavy chain (subclasses)
7
(
7 1
,
7 2
,
7 3
)
U
a l,a 2 3
ß
Ô
Light chain
Additional chains
K or A
Kor A
J
Kor A
J, SC
Kor A
Kor A
Valency (binding capacity)
2
5(10)4
2 (4, dimer)
2
2
Serum concentration, g/L)5
5.65-17.65
0.5-3
0.4 -3.5
<3 x IO“5
>0.01
%
plasma immunoglobulin
80
7-10
7-15
0.2
0.2
Rate of synthesis g per
2.2
0.5
1.5
0.01
0.03
70 kg person per day
Plasma half-life
20
5
6
2
3
Carbohydrate (wt %)
3
-12
~7
-12
-13
Complement fixation
Classical pathway
Y (IgGl-3)
Y
N
N
N
Alternative pathway
N
N
Y
N
N
Biological roles (selected)
Defends against
First antibody
Major immuno-
Causes
Membrane-bound,
microbes, crosses
formed to
globulin in
histamine
regulates and
placenta, binds
microbial
secretions
release from
triggers
to receptor or
antigens, in
defends
mast cells,
antibody
surface of
phagocytes
monomeric
form, B-cell
receptor
external
surfaces6
defense of
external
surfaces
synthesis
1 Number of 4-chain units in class.
2 Sedimentation coefficients are used in the older literature.
3a 2 does not have the inter-chain disulfide bonds.
4Steric effects appear to restrict binding.
5Ranges are from Tietz, N.W.,
C linical G u id e to L a b o ra to ry Tests,
W.B. Saunders, Philadelphia (1995).
6 External surfaces include respiratory, digestive and genitourinary tract surfaces.
the heavy chain sequences. The molecules of the im-
munoglobulin classes are as follows: IgG has
y
(gamma);
IgM
yt
(mu); IgA
a
(alpha); IgE
fi
(beta); and IgD
8
(delta)
heavy chains. Within the
y
chain are three subclasses
y
I,
y
2, and
y
3, and within the
a
chain are two subclasses
a \
and
a2.
Two classes of light chains also exist,
k
(kappa)
and
X
(lambda), which also possess different epitopes. An
individual immunoglobulin molecule contains either
k
or
X
light chains, but not both. Classes and subclasses are
described more fully in Table 35-2. Schematic structures
of the molecules of the immunoglobulin classes are shown
in Figure 35-11.
The five classes of immunoglobulin molecules are func-
tionally distinct, even when they recognize the same
epitopes. IgM, which contains five “basic” four-chain
immunoglobulin G-like structures, is decavalent, i.e., it
possesses the capacity to bind 10 antigen molecules.
Only two antigen molecules can bind to the two arms
of an IgG molecule, i.e., the IgG is bivalent. As result,
IgM molecules are more effective than IgG in binding
antigen, although generally IgM molecules bind anti-
gens more weakly than IgG molecules. IgM is the first
immunoglobulin synthesized after B-cell activation and
proliferation of plasma cells.
IgM
molecules,
in a
monomeric form, are the antigen receptors on B cells.
B-cell membrane-bound IgD antibodies also are involved
in the regulation of antibody synthesis. IgA molecules are
present in the mucous secretions of the respiratory, gas-
trointestinal, and genitourinary tracts. IgE is the antibody
formed in response to allergenic substances. It is responsi-
ble for the release of histamine from mast cells, the cause
of the discomfort associated with allergies.
