The Collaborative International Dictionary of English v.0.48:

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.]