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IgM
Immunoglobulin M is made of five of these "Y" shape
molecules. IgM is a pentameric molecule. The five are attached
to each other at the ends of the Fc regions so that the receptor
regions fan out through 360 like a cart wheel. This gives each
IgM molecule ten antigen binding sites. Inevitably IgM is considerably
heavier and bulkier than an IgG molecule weighing in at 970 kilodaltons.
Its size means that it is much more limited in its ability to
percolate through tissue. It's so big that it has difficulty even
penetrating through the walls of blood vessels. Much of IgM antibody
never makes it through the blood vessel endothelial wall and continues
to circulate through the blood stream until it is broken up. About
10% of all antibodies produced are IgM type antibodies.
In addition B cells also produce single IgM "Y" shaped
molecules to express on their cell surface. These along with IgD
antibodies act as antigen receptors for each B cell.
Basic IgM shape. IgM molecules consist
of five "Y" shapes linked together at their Fc regions.
IgA
Up to 20% of antibodies made are of the IgA type. For the most
part IgA is just one "Y" shape molecule but sometimes
two, three, four or five are joined together usually at the Fc ends
to form a dimers, trimers, tetramers and pentamers. There are two
subtypes of IgA molecule IgA1 and IgA2.
As well as being present in the blood stream, IgA is the type
of antibody found in body fluids such as saliva, milk, mucus and
colostrum. Most secretory IgA molecules are dimeric with a weight
of 390 kilodaltons. A minority of secretory IgA molecules are made
of tree, four or even six "Y" structures. Secretory IgA
molecules are made by the B cells we discussed found under the mucosal
surfaces in the gut, lungs and elsewhere. These B cells are diffusely
distributed or may come together temporarily in nests. The IgA molecules
are passed from the B cells straight through the epithelium and
out to the exterior to mix with the saliva, milk, mucus etc.
However, many if not all secretions contain various enzymes that
could attack the glycoprotein antibody. To prevent this secreted
IgA Has an extra piece of glycoprotein bolted on to it as the IgA
passes through the epithelium. Epithelial cells have the ability
to make this glycoprotein called the "secretory component".
It would seem that the piece of protein wraps around the weakest
point of the IgA molecule to stop any enzyme from physically getting
close to the weak area.
Basic IgA shape. Typically IgA is a dimer
of two "Y" molecules. They can link together end to
end or sometimes links arms.
IgD
Immunoglobulin D is a comparatively rare antibody type free floating
in the blood, only 1% of circulating antibody is type IgD. However,
IgD plays an important role. Each B lymphocyte needs to be able
to recognize antigen it can respond to. To do this each B cell produces
large quantities of IgD that remains attached to the cell surface.
This IgD acts as a cell receptor feeling for the appropriate antigen
that will fit in the variable cup region at the ends of the antibodies.
IgD is produced as a single "Y" shaped molecule both
when attached to B cell surfaces and when released to float free
in the blood. Each IgD molecule weighs about 185 kilodaltons and
is very rich in carbohydrate with up to 14% of the structure carbohydrate
based. The actions of free IgD is not known.
IgE
Immunoglobulin E is extremely hard to find in blood serum. It
only accounts for about 0.001% of free floating antibody in the
blood stream. However, IgE preferentially binds to basophils and
mast cells and everyone has these cells coated with large numbers
IgE antibodies. IgE is produced in a single form and weighs about
188 kilodaltons.
IgE is important in our defense against infection to helminthic
parasites (worms). IgE is important protection in underdeveloped
countries of the world where helminths proliferate in unsanitary
conditions but in the developed world IgE has developed an unwanted
role in hypersensitivity/atopic/allergic conditions.
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