A protein of the globulin-type found in serum or other body fluids that possesses antibody activity. An individual Ig molecule is built up from two light (L
) and two heavy (H
) polypeptide chains linked together by disulfide bonds. Ig
s are divided into five classes based on antigenic and structural differences in the H chains.
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 157 (https://doi.org/10.1351/pac199264010143)