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Introduction
You may remember from our look at the history of immunology
that the word immunoglobulin (Ig) was intentionally used to be
a general coverall term for the factors serum that apparently
responded to infectious agents in the simple transfer experiments
carried out by early immunologists. We now know these serum factors
capable of defense against pathogens are antibodies. The word
immunoglobulin is still used and for our purposes we can say that
an immunoglobulin is the equivalent of an antibody. Some immunologists
will argue that the word immunoglobulin covers more than just
antibodies but we will not complicate matters by going in to the
details here. All will be explained later.
We know from previous chapters that antibodies are made by B
cells generally once they are activated by a combination of recognizing
the presence of antigen and stimulation by T helper cells. We
know that antibody producing cells, known as blast cells, do not
migrate to the site of infection. They have no need to. Rather
they reside in central organs of the immune system such as the
spleen and lymph nodes where they can keep in constant communication
with other immune cells and receive feedback on the status of
the antigenic challenge. Changes in the messages the B cells receive
will result in an alteration in the activity of the B cells and
this response is fundamental to the adaptive immune system.
Antibodies pervade almost every tissue and fluid in our body.
The intelligent, long range missiles have been programmed to home
in on only a certain specific antigen and for the most part antibodies
are successful in destroying their target. Antibodies are small
glycosylated proteins (glycoproteins) and each healthy individual
has vast quantities of these glycoproteins floating around. In
each milliliter of blood serum there are up to 10,000,000,000,000,000
immunoglobulins. In this immunoglobulin population there are around
1,000,000 subgroups of immunoglobulins each group able to target
a different antigen. The concentration of antibody belonging to
a subgroup varies depending on the body's health status. Obviously
when under challenge from a pathogen the subgroups of immunoglobulins
able to target the pathogens antigens will expand in volume. Overall,
these figures are enough to defend us against almost any infectious
agent.
Antibody
basic structure
While there are several different classes of antibody and numerous
antigen specific subgroups, immunoglobulins all have a common basic
structure. First hypothesized by R Porter in 1962, each immunoglobulin
molecule is made up of 4 polypeptide chains. There are two long
chains, called the "heavy" or "H" chains which
weigh between 50 and 75 kilodaltons and two short chains called
"light" or "L" chains weighing in at 25 kilodaltons.
They are linked together by what are called disulfide bonds to form
a "Y" shape molecule.
The top two tips of the "Y" shape are the antigenic
binding sites so each antibody can bind two antigens. So an antibody
is described as "bivalent", it has two binding sites.
This is very important as we will see below. The four polypeptide
chains are arranged such that one light chain lies along side one
heavy chain and that the space between each heavy and light chain
combination forms a cup-like structure at the end. This cup form
will be the mirror image for part of an antigen. Very much like
a jigsaw puzzle piece, The antibody cup fits around part of an antigen.
In the same way each jigsaw piece will only properly attach to
its neighbor, so each immunoglobulin is shaped only to attach to
one type of antigen. The ends of the four polypeptide chains at
the tops of the "Y" shape are highly variable in their
molecular arrangement. Changes in the molecules change the shape
of the cup binding site they form. Of course any change in the shape
of the cup means that the appropriate antigen peptide that will
fit the cup will change.
The basic structure of all antibodies involves
four polypeptide chains linked together by disulfide bonds into
a "Y" shape. Some antibodies have more disulfide bonds
than others but the format is essentially the same.
Structural
regions of antibodies
So far we have mentioned that there are four polypeptide chains
that make up an antibody but we can also look at the different parts
of an antibody in terms of the structure these polypeptide chains
form. The structure of antibodies can be broken down into subcomponents.
Breaking up an antibody can be physically done using enzymes.
Dr Porter used this method to help him analyze antibody structure
and lead him to his hypothesis. He used the enzyme papain (comes
from the latex of Carica papaya plants) to split antibodies into
three pieces each of a similar size, shape and weight of about 45
kilodaltons. When looking at the "Y" shape of an antibody
we can see that it is really made up of three straight lines, one
for the stem and two branches. Papain splits the "Y" shape
at the junction between the straight lines to produce three separate
molecules. The two of the molecules that formed the branch of the
"Y" retain their ability to bind to antigen and are called
"Fab" fragments (Fab = fragment antigen binding). Because
they are now separate from each other they are described as "univalent".
The stem is the third molecule and is called the "Fc"
fragment (Fc = fragment crystalline).
Another enzyme, pepsin can also be used to break up antibody in
a slightly different way. Pepsin breaks an antibody lower down on
the stem of the "Y" a little bit below the papain cleavage
site. This results in two molecules. One is the Fc stem portion
again and the second molecule is the two branches of the "Y"
still joined together. This molecule is bivalent and is called an
Fab2 fragment.
Two experiments using the enzymes papain
and pepsin helped work out the shape and number of receptor sites
available on antibodies.
There is another method of breaking down the antibody into its
four constituent polypeptide chains. The chains are bound together
by disulfide bridges. More disulfide bridges are used to connect
each chain into a 3-D structure involving loops of protein called
"globules" hence the term immunoglobulin. Mercaptoethanol
(nasty chemical) can be used to break these disulfide bonds and
unravel an antibodies structure into the four polypeptide chains.
Papain, pepsin and Mercaptoethanol were the three products used
by Porter and others to define antibody structure. By using the
three in different combinations you end up with different antibody
pieces and polypeptide chains. It was a matter of analyzing the
different antibody fragments for their size and shape and then working
out how they fit together.
Types
of antibody
Above we have described the basic structure of all types of antibody
which involves the linking together of four polypeptide chains,
two heavy and two light chains. In humans there are five distinct
classes of antibody that can be differentiated by differences in
the general structure and conformation of these heavy and light
chains. In humans there are five main classes of immunoglobulin,
IgG, IgM, IgA, IgD and IgE. Antibody is identified as belonging
to one of the classes by analyzing differences in the heavy chains
molecular composition.
IgG
Immunoglobulin G (IgG) is the most common form of antibody produced.
About 75% of all immunoglobulins in our body are IgG. Compared to
the other classes of immunoglobulin it is small and light weighing
in at about 146 kilodaltons. Its heavy chains are called gamma heavy
chains and contain very little carbohydrate (about 3%). We can actually
subdivide this class into four subclasses designated IgG1, IgG2,
IgG3 and IgG4. IgG1 is the most common constituting 70% of all IgG
produced and IgG4 is the least common with only 2% of all IgG being
subtype IgG4. All the subtypes are very similar to each other in
their structure. It is only a matter of a few extra molecular links
in the chains which differentiates the subtypes. However the different
subtypes have very different properties. For example, IgG1 and IgG2
are good a binding complement but IgG3 and IgG4 are not.
Basic IgG shape.
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