T cell immunogens are proteins. Haptens are small molecules that are not immunogenic, but can be antigenic. When haptens are conjugated to carrier proteins, the hapten-carrier protein is immunogenic, and the immunity that results will be against the carrier protein, the hapten, and the combination of the two. Examples of haptens include steroids such as testosterone, and oils such as pentadecacatechol from poison oak, and drugs such as penicillin. The portion of the immunogen that the cells bind to is called an epitope or antigenic determinant.
Most immunogens have several, overlapping epitopes.
The size of a B cell epitope is determined by the size of the antigen-binding site of the antibody molecule displayed by the B cell. T cell epitopes are linear sequences of amino acids, usually 10-20 amino acids in length. Molecules with a mass less than 5000 dalton are usually poor immunogens. The best immunogens have masses around 100,000 dalton. Presumably, larger molecules are retained longer in the body, are more likely to be bound by complement or trapped in lymphoid follicles, and are more likely to be phagocytosed or endocytosed for presentation on MHC to T cells.