Antibody Molecules, Genetic Engineering of
333
IgG
IgD
IgA
J-chain
IgE
IgM
J-chain
Fig. 2
The fve classes oF antibodies: IgG, IgD, and IgE are monomeric antibodies; IgA and
IgM are polymeric antibodies.
antibody. Figure 2 shows the fve di±±erent
classes o± antibodies, which di±±er in the
constant region o± their respective heavy
chains. The di±±erent heavy-chain isotypes
that defne these classes o± antibodies are
designated by lower-case Greek letters:
γ
±or IgG,
δ
±or IgD,
ε
±or IgE,
α
±or
IgA, and
µ
±or IgM. IgM and IgE heavy
chains contain an extra C
H
domain (C
H
4)
and lack the hinge region ±ound in IgG,
IgD, and IgA. However, the absence o± a
hinge region does not imply that IgM and
IgE lack flexibility; electron micrographs
o± IgM molecules binding to ligands have
shown that the Fab arms can bend, relative
to the Fc ±ragment. There are also two
di±±erent light-chain isotypes, which are
designated by the lower-case Greek letters
κ
and
λ
. Light chains o± both isotypes
are ±ound associated with all the heavy-
chain isotypes.
In humans, there is only one class o±
IgD, IgE, and IgM with monomeric struc-
tures having a molecular weight (MW) o±
around 190 kDa. However, human IgA an-
tibodies can be ±urther subdivided in two
subclasses (IgA1 and IgA2) with a molec-
ular weight o± around 160 kDa ±or each
monomer, and human IgG antibodies can
be subdivided into ±our subclasses: IgG1,
IgG2, IgG4 (all three with a molecular
weight o± around 150 kDa) and IgG3 with
a molecular weight o± around 165 kDa.
The higher molecular weight exhibited by
human IgG3 is due to the presence o± an
extended hinge region, which provides ex-
traordinary flexibility. In ±act, IgG3 is the
most flexible human IgG.
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