Antigens are basic molecules that induce an immune response when detected by immune system cells. Antigens may be either complete or incomplete based on the nuances of their molecule structure.

Haptens

A hapten is essentially an incomplete antigen. These small molecules can elicit an immune response only when attached to a large carrier such as a protein; the carrier typically does not illicit an immune response by itself. Many hapten carriers are normal molecules that circulate through the body. When haptens and carriers combine, the resulting molecule is called an adduct, the combination of two or more molecules. Haptens cannot independently bind to MHC complexes, so they cannot be presented to T cells.

The first haptens used were aniline and its carboxyl derivatives (o-, m-, and p-aminobenzoic acid). One well-known hapten is urushiol, the toxin found in poison ivy and a common cause of cell-mediated contact dermatitis. 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 causes only sensitization, in which there is a proliferation of helper and cytotoxic T cells. After a second exposure, the proliferated T cells can become activated, generating an immune reaction and producing the characteristic blisters of poison ivy exposure.

Some haptens induce autoimmune disease. An example is hydralazine, a blood pressure-lowering drug that occasionally causes lupus erythematosus (an autoimmune inflammatory disorder) in certain individuals with genetic predispositions to the disease. This also appears to be the mechanism by which the anesthetic gas halothane can cause life-threatening hepatitis and penicillin-class drugs cause autoimmune hemolytic anemia. Other haptens, such as flourescein, detect proteins with which they form adducts. This makes them a common part of molecular biology lab techniques.

Complete Antigens

A complete antigen is essentially a hapten-carrier adduct. 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 will usually not initiate an immune response. In most cases this can only be elicited by theonly the hapten-carrier adduct. Sometimes the small-molecule hapten can block immune response to the complete antigen by preventing the adduct from binding to the antibody, a process called hapten inhibition. In this case, the hapten acts as the epitope for the antigen, which binds to the antibodies without causing a response. If this happens with enough haptens, there will not be enough antibodies left to bind to the complete antigen, thus inhibiting the antibody response.

Fluorescein Molecule

Fluorescein is an example of a hapten used in molecular biology.