634
UNIT FOUR
Constant regions impart other properties of the antibody
molecule, such as its ability to bond to cellular structures or
to combine with certain chemicals (F
g. 16.20).
Types of Immunoglobulins
Of the F
ve major types of immunoglobulins, three constitute
the bulk of the circulating antibodies. They are immuno-
globulin G, which accounts for about 80% of the antibodies;
Antibody Molecules
Antibodies are soluble, globular proteins that constitute the
gamma globulin
fraction of plasma proteins (see chapter 14,
p. 537). Each antibody molecule consists of four chains of
amino acids linked by pairs of sulfur atoms that attract by
disulF
de bonds. The four chains form a Y-shaped structure
(f g. 16.20)
. Two of these amino acid chains are identical
light chains
(L-chains), and two are identical
heavy chains
(H-chains). The heavy chains have about twice as many
amino acids as the light chains. The F
ve major types of anti-
body molecules are distinguished by a particular type of
heavy chain. Most of the types of antibody molecules consist
of a single Y-shaped structure, but some have as many as
F ve (see F
g. 14.21).
As with other proteins, the sequences of amino acids
of the heavy and light chains confer the unique, three-
dimensional structure (conformation) of each antibody.
This special conformation, in turn, imparts the physiological
properties of the molecule. ±or example, one end of each of
the heavy and light chains consists of variable sequences of
amino acids (variable regions). These regions are specialized
to F
t the shape of a speciF
c antigen molecule.
Antibodies can bind to certain antigens because of the
conformation of the variable regions. The antibody contorts
to form a pocket around the antigen. These specialized
ends of the antibody molecule are called
antigen-binding
sites,
and the parts that bind the antigen are called
idio-
types
(id
e-o-tı¯pz
).
The remaining parts of the chains are termed constant
regions because their amino acid sequences are similar.
TABLE
16.6
|
Steps in Antibody Production
B Cell Activities
1. Antigen-bearing agents enter tissues.
2. B cell encounters an antigen that f
ts its antigen receptors.
3. Either alone or more oFten in conjunction with helper T cells, the B cell is activated. The B cell proliFerates, enlarging its clone.
4. Some oF the newly Formed B cells di±
erentiate Further to become plasma cells.
5. Plasma cells synthesize and secrete antibodies whose molecular structure is similar to the activated B cell’s antigen receptors.
6. Antibodies combine with antigen-bearing agents, helping to destroy them.
T Cell Activities
1. Antigen-bearing agents enter tissues.
2. An accessory cell, such as a macrophage, phagocytizes the antigen-bearing agent, and the macrophage’s lysosomes digest the agent.
3. Antigens From the digested antigen-bearing agents are displayed on the membrane oF the accessory cell.
4. Helper T cell becomes activated when it encounters a displayed antigen that f
ts its antigen receptors.
5. Activated helper T cell releases cytokines when it encounters a B cell previously combined with an identical antigen-bearing agent.
6. Cytokines stimulate the B cell to proliFerate.
7. Some oF the newly Formed B cells give rise to cells that di±
erentiate into antibody-secreting plasma cells.
8. Antibodies combine with antigen-bearing agents, helping to destroy them.
S-S
Antigen-binding
sites
Disulfide bonds
S-S
Variable region
of light chain
Constant region
of light chain
Variable region
of heavy chain
Constant region
of heavy chain
S-S
S-S
FIGURE 16.20
An antibody (immunoglobulin) molecule consists
basically oF two identical light chains oF amino acids and two identical
heavy chains oF amino acids.
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