P-phenylenediamine

P-phenylenediamine (PPD) is a strong contact allergen which is used in oxidative hair dyes and may cause severe allergic contact dermatitis. Although PPD has been recognized as a strong contact allergen for many years, the exact mechanisms of sensitization and elicitation are not yet elucidated. Penetration and the ability to bind to proteins are the first two hurdles an allergen has to overcome to be able to sensitize. These processes are described in a detailed review of the current literature. Additionally, the real-time penetration into the living human skin has been studied using a unique, non-invasive spectroscopic method. Already thirty minutes after application a sufficient amount of PPD had penetrated the skin from a diagnostic PPD patch test, and the patch becomes depleted within 24 hours, whereas the current diagnostic patch test application time is 48 hours.

The influence of detoxification enzymes on the induction of contact sensitization was studied in a case-control study, which showed that polymorphisms in the glutathione-S-transferase genes had a disease-modifying effect with a weak biological impact. A European Consortium was created to be able to establish a genome-wide association analysis. An elicitation study taught us that there is a dose-response relationship, in which with (in vitro determined) true exposure levels an individual range of elicitation thresholds could be determined. A final study, of importance to patients with a PPD allergy and a persistent wish to dye their hair, demonstrated that the new hair dye may be a safe alternative, although 30% of the PPD allergic patients also reacted to the new hair dye.