Biology of Disease

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