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IMMUNOGLOBULIN
(Ig): (Ig) Also called immune serum globulin. A class of proteins
also known as antibodies made by the B cells of the immune system in response
to a specific antigen, and containing a region that binds to this antigen
(antigen-binding site). Each immunoglobulin is made up of two polypeptides,
a heavy chain and a light chain. These heavy and light chains complex to
form the antigen binding site. Each group of immunoglobulins is named for
the type of heavy chain that it contains. The heavy chain determines the
post-antigen binding activity of an antibody. There are five classes of
immunoglobulins: IgA, IgD, IgE, IgG, and IgM.
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IMMUNOGLOBULIN
A (IgA): (IgA) A class of antibodies, often formed as a dimer (i.e.,
two antibody molecules attached to each other end to end), that is secreted
into bodily fluids such as saliva. IgA protects the body's mucosal surfaces
from infections.
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IMMUNOGLOBULIN
D (IgD): (IgD) A class of antibodies that is present in low concentration
in serum. The primary function of IgD appears to be as an antigen receptor
on mature B cells.
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IMMUNOGLOBULIN
E (IgE): (IgE) A class of antibodies involved in anti-parasite (see
parasite) immunity and in allergies.
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IMMUNOGLOBULIN
G (IgG): (IgG) A class of antibodies composed of two identical light
and two identical heavy polypeptide chains. IgG acts on antigens by agglutinating
(clumping cells together) them. In pregnancy, IgG crosses the placenta
to the fetus and protects it against red cell antigens and white cell antigens.
Also called gamma globulin.
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IMMUNOGLOBULIN
M (IgM): (IgM) A class of antibodies that is made by the body as the
initial response to an antigen. If IgM is made in response to a vaccination,
a booster shot will result in a "switch" from IgM to mostly immunoglobulin
G.
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