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.