Antibody

• a substance engendered in an organism by the presence of a foreign material, especially bacterial proteins; an antibody is specifically antagonistic to the antigen or substance under whose influences it was formed
  Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby
• also called an immunoglobulin, it is a protein produced by a lymphocyte in response to exposure to an antigen and reacts very specifically with that antigen.
  Source: Jenkins, John B. 1990. Human Genetics, 2nd Edition. New York: Harper & Row