Hapten
Generally, only large molecules, infectious agents, or insoluble foreign matter can elicit an immune response in the body.A hapten is a small molecule which can elict an immune response only when attached to a large carrier such as a protein; the carrier may be one which also does not elicit an immune response by itself. 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 usual not initiate an immune response; usually only the hapten carrier adduct can do this. Sometimes the the small-molecule hapten can even block immune response to the hapten-carrier adduct by preventing the adduct from binding to the antibody.
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. Typicaly the first exposure only causes sensitization, in which there is a proliferation of B cells able to make antibody to the hapten adduct. After a second exposure later, the proliferated B 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 which occasionally can produce drug-induced lupus erythematosus in certain individuals.