antibody
A protein (immunoglobulin) produced by the immune system of an organism in response to exposure to a foreign molecule (antigen) and characterized by its specific binding to a site of that molecule (antigenic determinant or epitope).
Source:
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 146
PAC, 1993, 65, 2003 (Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)) on page 2014