Haptens (derived from the Greek haptein, meaning “to fasten”) are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself. The mechanisms of absence of immune response may vary and involve complex immunological interactions, but can include absent or insufficient co-stimulatory signals from antigen-presenting cells.

Haptens have been used to study allergic contact dermatitis (ACD) and the mechanisms of inflammatory bowel disease (IBD) to induce autoimmune-like responses.

The concept of haptens emerged from the work of Austrian immunologist Karl Landsteiner, who also pioneered the use of synthetic haptens to study immunochemical phenomena.