What is the role of hapten allergies present in cosmetics on eczema?

An hapten is a chemical substance of low molecular weight that can be considered as a partial antigen. Indeed, a hapten alone does not trigger the formation of antibodies by the body but provokes it when it combines with another molecule, referred to as a carrier. This carrier is often a protein. Therefore, a hapten possesses the property of reactivity, as it can associate with another molecule, but not that of immunogenicity, as it is incapable of inducing an allergic reaction if it is alone. Haptens are generally classified according to their sensitising power.

• Strong haptens.

  Strong haptens are chemical substances that sensitise over 90% of individuals upon simple contact. They are generally absent from our everyday environment, an example being 2,4-dinitrochlorobenzene (DNCB). They are often used in experimental models to further the pathophysiological study of eczema. The sensitising power of strong haptens is proportional to their pro-inflammatory capacity. There is no immune tolerance to strong haptens and eczema develops upon their first contact with the skin.

• Moderate haptens.

  Moderate haptens are quite commonly found in our everyday environment. They induce sensitisation in 1 to 20% of individuals when they come into contact with the skin. Nickel, for instance, which is present in some jewellery, is a prime example.

• Weak haptens.

  Weak haptens are thousands of allergenic molecules found in our everyday environment. These include dyes, preservatives, and fragrances present in cosmetic or medical care products. Weak haptens are also found in certain industrial chemicals, such as solvents, adhesives, and paints. They are mildly pro-inflammatory and cause sensitisation in less than 1% of individuals.

Contacteczema is characterised by red, swollen patches accompanied by intense itching. This condition occurs after repeated skin contact with a substance it cannot tolerate, known as a hapten. Being hydrophobic and of low molecular weight, haptens have a good ability to penetrate the skin transcutaneously. Skin that is chronically damaged by irritations or has a genetic deficit in the skin barrier (atopy) promotes the penetration of haptens and thus the onset of contact eczema. It is also possible to have contact eczema on top of atopic eczema.

From a biological perspective, once in contact with the skin, the hapten binds to certain epidermal proteins. It then acquires the property of immunogenicity and becomes an antigen. This hapten-carrier complex is recognised by B lymphocytes, which are antigen-presenting cells (APCs) belonging to the immune system. The antigen is then endocytosed, meaning it is incorporated into the APCs, before being fragmented. One of these fragments, called a peptide antigen, is bound by intracytoplasmic molecules such as MHCs, the major histocompatibility complexes. These MHCs play a crucial role in self-recognition and are located on the surface of the APCs. Finally, the antigen carried by the MHC is recognised by the receptors of T lymphocytes (TCR) Helper CD4+ of type Th1 and Th2, which, when activated, release cytokines, thereby causing inflammation. The activated T lymphocytes then migrate to the lymph nodes where they multiply to form a population of sensitised T lymphocytes.

Depending on the sensitising power of the hapten, the inflammatory reaction does not necessarily occur immediately after the first contact with the skin. This can take several hours, or even a second exposure. Indeed, upon re-exposure to the hapten, the sensitised T lymphocytes react more strongly and the release of cytokines and other inflammation mediators is greater than before. That's why it's necessary to avoid the offending substance to prevent recurrences in cases of contact eczema. To identify it, we recommend you consult a dermatologist or allergist who, with the help of a series of patch tests, can determine which element you are sensitive to.

• GIORDANO-LABADIE F. Eczéma de contact et dermatite atopique de l’enfant : les haptènes. Revue française d’allergologie (2013).

• SAURAT J. H., LACHAPELLE J. M., LIPSKER D., THOMAS L. et BORRADORI L. Dermatologie et infections sexuellement transmissibles. Elsevier Masson (2017).