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The Collaborative International Dictionary of English v.0.48 (gcide)

Antibody \An"ti*bod`y\ ([a^]n"t[i^]*b[o^]d`[y^]), n.
1. (Med., Physiol. Chem.) Any of various bodies or substances
in the blood which act in antagonism to harmful foreign
bodies, as toxins or the bacteria producing the toxins.
Normal blood serum apparently contains various antibodies,
and the introduction of toxins or of foreign cells also
stimulates production of their specific antibodies by the
immune system.

Note: In certain "autoimmune" diseases, the immune system of
an organism may produce antibodies to parts of the
organism's own structure, and can cause considerable
damage and even death as a result.
[Webster 1913 Suppl. PJC]

2. (Med., Physiol. Chem.) more narrowly, any of the
immunoglobulins present in the blood serum or other body
fluids of an animal, which reacts with a specific
antigenic substance, whether the antibody was produced as
a consequence of the stimulus provided by the antigen, or
was pre-existing prior to exposure of the organism to the
antigen.

Note: The soluble antibodies present in serum are complex
globins (proteins) with both constant and variable
regions in the polypeptide chains, the variable region
being responsible for the reaction of the antibody with
its specific antigen. Typically, antibodies of more
than one structure may react with a given antigen, and
any given antigen may stimulate production of more than
one antibody. Methods have been developed to grow cells
in tissue culture which produce predominantly only one
specific antibody, and such antibodies thus produced
(called {monoclonal antibodies}) are highly specific in
their reactions to molecular strucures, and are used as
valuable reagents in biochemical technology.
[PJC.]