Hapten

A small molecule which, although not itself, can trigger an immunological response when bound to a protein. This protein is called a carrier protein or "carrier". Haptens can be inorganic substances, lipids, nucleic acids, drugs (e.g. penicillins), etc. The carrier protein can be found in the skin, on the cell membrane of a red blood cell or platelet, etc.

The hapten/prohapten hypothesis is supplemented by the so-called p-i concept (pharmaceutical interaction with immunreceptors). This concept states that drugs can also interact directly with cell surface receptors, e.g. with T-cell receptors, and thus activate T-cells.