hapten

A low-molecular weight molecule which contains an antigenic determinant but which is not itself antigenic unless complexed with an anitgenic 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