296
UNIT IV
Infection, Inflammation, and Immunity
IgM
accounts for approximately 10% of all circulating
antibodies. It normally exists as a pentamer with identical
heavy chains and identical light chains. Because of its struc-
ture, it is an efficient complement fixing Ig and is instrumental
in the ultimate lysis of microorganisms. It also functions as an
effective agglutinating antibody, capable of clumping organ-
isms for eventual lysis and elimination. IgM is the first anti-
body to be produced by the developing fetus and by immature
B lymphocytes.
IgD
is a monomer found primarily on the cell membranes
of B lymphocytes where it functions as a receptor for antigen.
It circulates in the serum in extremely low levels where its
function is essentially unknown. IgD on the surface of B lym-
phocytes contains extra amino acids at C-terminal so that it
can successfully anchor to the membrane. It also associates
with the Ig-alpha and Ig-beta chains.
IgE
is the least common serum IgE because it binds very
tightly to the Fc receptors on basophils and mast cells. It is
involved in inflammation and allergic responses by causing
mast cell degranulation and release of chemical mediators
including histamine. IgE is also essential for combating para-
sitic infections.
TABLE 13.4 CLASSES AND CHARACTERISTICS OF Igs
FIGURE
CLASS
PERCENTAGE OF TOTAL
CHARACTERISTICS
IgG
75.0
Displays antiviral, antitoxin, and antibacterial properties; only
Ig that crosses the placenta; responsible for protection of
newborn; activates complement and binds to macrophages
IgA
15.0
Predominant Ig in body secretions, such as saliva, nasal and
respiratory secretions, and breast milk; protects mucous
membranes
IgM
10.0
Forms the natural antibodies such as those for ABO blood
antigens; prominent in early immune responses; activates
complement
IgD
0.2
Found on B lymphocytes; needed for maturation of B cells
IgE
0.004
Binds to mast cells and basophils; involved in parasitic infec-
tions, allergic and hypersensitivity reactions
and is composed of two different kinds of polypeptide chain.
IgG possesses antiviral, antibacterial, and antitoxin properties.
It is present in all body fluids, readily enters the tissues, and
is capable of crossing the placenta where it confers immu-
nity upon the fetus. Intact IgG functioning requires the help
of APCs. It binds to target cells as well as Fc receptors on
NK cells and macrophages, leading to lysis of the target cell.
There are four subclasses of IgG (
i.e.,
IgG1, IgG2, IgG3, and
IgG4) with specificity for certain types of antigens. For exam-
ple, IgG2 appears to be responsive to bacteria that are encap-
sulated with a lipopolysaccharide layer, such as
Streptococcus
pneumoniae
,
Neisseria gonorrhoeae
, and several strains of
Salmonella
.
30
IgA
possesses a dimeric structure and is the second most
common Ig found in serum accounting for approximately
15% of all antibodies. It is primarily a secretory Ig that is
found in saliva, tears, colostrum (
i.e.,
first milk of a nursing
mother), and bronchial, gastrointestinal, prostatic, and vagi-
nal secretions. Because it is primarily found in secretions, its
primary function is in local immunity on mucosal surfaces.
IgA prevents the attachment of viruses and bacteria to epi-
thelial cells.
