Immunoglobulin G
Immunoglobulin G (IgG) is the most abundant antibody in the blood where it is
found at concentrations of approximately 13.5 mg cm–3. It is evenly distributed
between the vascular and extravascular compartments. In humans, IgG occurs
as four subclasses, all with slightly different properties and roles in the body.
For example, all subclasses except IgG 4 can activate complement (Table 4.3).
Immunoglobulin G is produced after IgM on exposure to an immunogen and
is the predominant antibody produced on a second exposure, that is in the
secondary response. It is also the only antibody in humans that can cross the
placenta to allow maternal antibodies to protect the developing fetus from
infection. On some occasions, this may cause problems as, for example, when
the mother has become sensitized to fetal antigens or has an autoimmune
disease (Chapter 5).
Immunoglobulin G is a symmetrical molecule consisting of four polypeptide
chains joined by disulfide bonds as shown in Figure 4.8(A). The four chains
SPECIFIC IMMUNE RESPONSES
CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY -&
Figure 4.8 The structure of IgG shown (A) diagrammatically, (B) molecular model PDB file
1HZH and (C) showing a bound antigen (black). (C) Courtesy of Dr R.S.H. Pumphrey, St Mary’s
Hospital, Manchester, UK.
B)
Class Subclasses Mr Heavy
chains
Activates
complement
Crosses
placenta
Opsonization Triggers inflammation
IgM none 900 000 L yes no no indirectly via complement
IgG IgG 1
IgG 2
IgG 3
IgG 4
146 000
145 000
170 000
146 000
F 1
F 2
F 3
F 4
yes
yes
yes
yes
yes
yes
yes
yes
- yes
yes
indirectly via complement
IgA IgA 1
IgA 2
160 000 to
380 000
@ 1
@ 2
no
no
no
no
yes
yes
no
no
IgE none 188 000 D no no no directly by binding toDreceptors on mast cells
IgD none 184 000 C no no no no
Table 4.3Properties of immunoglobulin classes. Light chains are always eitherJorK.
C)
Fab
Fab
S
S
S
S
S
S
L
H
H
L
COO-
COO-
COO-
COO-
Fc
A)
H 3 N
H 3 N
H 3 N
H 3 N