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).
Sources:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 146 (https://doi.org/10.1351/pac199264010143)
PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2014 (https://doi.org/10.1351/pac199365092003)