hapten

A hapten is a small molecule that is not immunogenic on its own but can become capable of eliciting an immune response when covalently attached to a larger carrier protein or polysaccharide. In such hapten-carrier complexes, B cells specific for the hapten can bind the complex, internalize it, and present carrier-derived peptides to helper T cells, which provide the necessary signals for antibody production and class switching. The immunogenic determinant, or epitope, is the hapten; the carrier provides T cell epitopes and the context needed for a robust response. Haptens are typically below a certain size threshold, often cited as under about 500–1000 daltons.

Common examples used in research include dinitrophenyl (DNP) and nitrophenyl haptens conjugated to proteins. In clinical settings, many drug allergies arise when small drug molecules such as penicillin act as haptens that form covalent adducts with host proteins, creating neoantigens that trigger T cell–mediated responses. Haptens are routinely conjugated to immunogenic carriers such as albumin, keyhole limpet hemocyanin (KLH), or ovalbumin to produce immunogenic conjugates for vaccines or diagnostic assays (for example, enzyme-linked immunosorbent assays).

In research and clinical immunology, haptens enable study of antibody specificity and hapten-carrier immunogenicity, and they underpin several diagnostic and therapeutic strategies. The concept explains why small molecules alone often fail to provoke immunity, yet can be targeted by antibodies when attached to larger carriers, shaping approaches to allergy testing, serology, and vaccine design.