A low-molecular weight molecule which contains an antigenic determinant but which is not itself antigenic unless complexed with an antigenic carrier. Examples of haptens are dinitrophenols, phosphorylcholine and dextran.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 156 (https://doi.org/10.1351/pac199264010143)