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A hapten is a small molecule that can 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. (In general, only large molecules, infectious agents, or insoluble foreign matter can elicit an immune response in the body.) Once the body has generated antibodies to a hapten-carrier adduct, the small-molecule hapten may also be able to bind to the antibody, but it will usually not initiate an immune response; usually only the hapten-carrier adduct can do this. Sometimes the small-molecule hapten can even block immune response to the hapten-carrier adduct by preventing the adduct from binding to the antibody.

The concept of haptens emerged from the work of Karl Landsteiner[1] [2] who also pioneered the use of synthetic haptens to study immunochemical phenomena.[3]


Examples of haptens

A well-known example of a hapten is urushiol, which is the toxin found in poison ivy. When absorbed through the skin from a poison ivy plant, urushiol undergoes oxidation in the skin cells to generate the actual hapten, a reactive molecule called a quinone, which then reacts with skin proteins to form hapten adducts. Usually, the first exposure only causes sensitization, in which there is a proliferation of effector T-cells. After a second exposure later, the proliferated T cells can become activated, generating an immune reaction, producing the typical blisters of poison ivy exposure.

Some haptens can induce autoimmune disease. An example is hydralazine, a blood pressure-lowering drug that occasionally can produce drug-induced lupus erythematosus in certain individuals. This also appears to be the mechanism by which the anaesthetic gas halothane can cause a life-threatening hepatitis, as well as the mechanism by which penicillin-class drugs cause autoimmune hemolytic anemia.

Other haptens that are commonly used in molecular biology applications include fluorescein, biotin, digoxigenin, and dinitrophenol.


  1. ^ Landsteiner, Karl (1945). The Specificity of Serological Reactions. Harvard Univ. Press. 
  2. ^ Landsteiner, Karl (1990). The Specificity of Serological Reactions, 2nd Edition, revised. Courier Dover Publications. ISBN 0486662039. 
  3. ^ Shreder, Kevin (March 2000). "Synthetic Haptens as Probes of Antibody Response and Immunorecognition". Methods (Academic Press) 20 (3): 372–379. doi:10.1006/meth.1999.0929. 

See also

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