336
U N I T 4
Infection and Immunity
Immunoglobulins
Immunoglobulins, or antibodies, function as antigen
receptors for B cells or as effector molecules of the
humoral immune response. The immunoglobulins have
been divided into five classes—IgG, IgA, IgM, IgD, and
IgE—each with a different role in the immune defense
strategy (Table 15-4). Immunoglobulins have a charac-
teristic four-polypeptide structure consisting of at least
two identical antigen-binding sites (Fig. 15-10). Each
Ig is composed of two identical light (L) chains and
two identical heavy (H) chains to form a “Y”-shaped
TABLE 15-4
Classes and Characteristics of Immunoglobulins
Figure
Class
Percentage of Total
Characteristics
IgG
75.0
Displays antiviral, antitoxin, and antibacterial
properties; only Ig that crosses the placenta
and thus responsible for protection of newborn;
activates complement and binds to macrophages;
prominent in the secondary immune response
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 infections and allergic and hypersensitivity
reactions
B-cell antigen
receptor on
surface of
immunoglobulin
Proliferation
TCR
TCR
CD4
Antigen MHC-II
B
cell
Plasma
cell
Differentiation
Antibody
B
cell
Memory
B
cell
B
cell
Cytokines
Helper
T cell
FIGURE 15-9.
Pathway for B-cell differentiation. MHC, major histocompatibility class;TCR,T-cell receptor.
