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e 2C Ribbon drawing
of the first intact antibody (IgG2A)
Harris, L. J., Larson, S. B., Hasel,
K. W., Day, J.,
Greenwood, A., McPherson, A. Nature 1992, 360, 369-372. © 2000
Glycoprotein molecules that are produced by plasma cells in
response to an immunogen and which function as antibodies. The
immunoglobulins derive their name from the finding that they
migrate with globular proteins when antibody-containing serum is
placed in an electrical field (Figure 1).
FUNCTIONS OF IMMUNOGLOBULINS
Immunoglobulins bind specifically to one or a few closely
related antigens. Each immunoglobulin actually binds to a
specific antigenic determinant. Antigen binding by antibodies is
the primary function of antibodies and can result in protection
of the host. The valency of antibody refers to the number of
antigenic determinants that an individual antibody molecule can
bind. The valency of all antibodies is at least two and in some
Frequently the binding of an antibody to an antigen has no
direct biological effect. Rather, the significant biological
effects are a consequence of secondary "effector functions" of
antibodies. The immunoglobulins mediate a variety of these
effector functions. Usually the ability to carry out a
particular effector function requires that the antibody bind to
its antigen. Not every immunoglobulin will mediate all effector
functions. Such effector functions include:
Fixation of complement - This results in lysis of cells and
release of biologically active molecules
Binding to various cell types - Phagocytic cells,
lymphocytes, platelets, mast cells, and basophils have
receptors that bind immunoglobulins. This binding can
activate the cells to perform some function. Some
immunoglobulins also bind to receptors on placental
trophoblasts, which results in transfer of the
immunoglobulin across the placenta. As a result, the
transferred maternal antibodies provide immunity to the
fetus and newborn
Figure 2A The basic structure of
STRUCTURE OF IMMUNOGLOBULINS
structure of the immunoglobulins is illustrated in the Figure 2.
Although different immunoglobulins can differ structurally they all
are built from the same basic units.
and Light Chains
All immunoglobulins have a four chain structure as their basic
unit. They are composed of two identical light chains (23kD) and
two identical heavy chains (50-70kD)
Inter-chain disulfide bonds - The heavy and light chains and
the two heavy chains are held together by inter-chain
disulfide bonds and by non-covalent interactions The number
of inter-chain disulfide bonds varies among different
Intra-chain disulfide binds - Within each of the polypeptide
chains there are also intra-chain disulfide bonds.
Variable (V) and Constant (C) Regions
After the amino acid sequences of many different heavy chains
and light chains were compared, it became clear that both the
heavy and light chain could be divided into two regions based on
variability in the amino acid sequences. These are the:
Chain - VL (110 amino acids) and CL
(110 amino acids)
Chain - VH (110 amino acids) and CH
(330-440 amino acids)
This is the region at which the arms of the antibody molecule
forms a Y. It is called the hinge region because there is some
flexibility in the molecule at this point.
Three dimensional images of the immunoglobulin molecule show
that it is not straight as depicted in Figure 2A. Rather, it is
folded into globular regions each of which contains an
intra-chain disulfide bond (figure 2B-D). These regions are
Chain Domains - VL and CL
Chain Domains - VH, CH1 - CH3
Carbohydrates are attached to the CH2 domain in most
immunoglobulins. However, in some cases carbohydrates may also
be attached at other locations.
OF THE VARIABLE REGION
Structure of the variable region framework
Hypervariable (HVR) or complementarity determining regions (CDR)
of the amino acid sequences of the variable regions of
immunoglobulins show that most of the variability resides in
three regions called the hypervariable regions or the
complementarity determining regions as illustrated in Figure 3.
Antibodies with different specificities (i.e. different
combining sites) have different complementarity determining
regions while antibodies of the exact same specificity have
identical complementarity determining regions (i.e. CDR
is the antibody combining site). Complementarity determining
regions are found in both the H and the L chains.
between the complementarity determining regions in the variable
region are called the framework regions (Figure 3). Based on
similarities and differences in the framework regions the
immunoglobulin heavy and light chain variable regions can be
divided into groups and subgroups. These represent the products
of different variable region genes.
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V. IMMUNOGLOBULIN FRAGMENTS:
fragments produced by proteolytic digestion have proven very useful
in elucidating structure/function relationships in immunoglobulins.
Digestion with papain breaks the immunoglobulin molecule in the
hinge region before the H-H inter-chain disulfide bond Figure 4.
This results in the formation of two identical fragments that
contain the light chain and the VH and CH1
domains of the heavy chain.
binding - These fragments were called the Fab fragments
because they contained the antigen binding sites of the
antibody. Each Fab fragment is monovalent whereas the
original molecule was divalent. The combining site of the
antibody is created by both VH and VL.
An antibody is able to bind a particular antigenic
determinant because it has a particular combination of VH
and VL. Different combinations of a VH
and VL result in antibodies that can bind a
different antigenic determinants.
Digestion with papain also produces a fragment that contains the
remainder of the two heavy chains each containing a CH2
and CH3 domain. This fragment was called Fc because
it was easily crystallized.
Figure 4 Immunoglobulin fragments:
Figure 5 Immunoglobulin
fragments: Structure function relationships
Effector functions -
The effector functions of immunoglobulins are mediated by
this part of the molecule. Different functions are mediated
by the different domains in this fragment (Figure 5).
Normally the ability of an antibody to carry out an effector
function requires the prior binding of an antigen; however,
there are exceptions to this rule.
Figure 6 Immunoglobulin fragments:
Treatment of immunoglobulins with pepsin results in cleavage of
the heavy chain after the H-H inter-chain disulfide bonds
resulting in a fragment that contains both antigen binding sites
(Figure 6). This fragment was called F(ab')2 because
it was divalent. The Fc region of the molecule is digested into
small peptides by pepsin. The F(ab')2 binds antigen
but it does not mediate the effector functions of antibodies.
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