Pathophysiological Study of Allergic Contact Dermatitis to Para-Phenylenediamine (PPD). Analysis of Cellular and Molecular Targets in Skin Inflammation

Inclusion Criteria: For both groups: * Patients aged from 18 to 65 years old. * Both genders eligible for study. * Female participants must use a contraceptive method. * Feasibility of patch testing. * Participants must be able to understand and sign the Informed Consent, and comply with all aspects of the protocol. * Patients must be registered in a social security system or with a health insurance coverage  First group: allergic patients * Patients with allergic contact dermatitis to para-phenylenediamine (PPD) based on a history of PPD contact dermatitis and positive PPD patch tests.  Second group : healthy volunteers * No history of PPD allergic contact dermatitis, with a negative PPD patch test. Exclusion Criteria: * Pregnant or lactating women. * Evolutive skin disease on the testing zone (lower back). * Patients with a clinically significant disease (chronic, recurrent or active). * Systemic corticotherapy or immunosuppressive treatment during the previous month, or local corticoid treatment the week before the patch testing. * Local or systemic drug use which interacts with the outcome measures. * Exposure to sun or UV radiations, 15 days before the patch testing. * Patients deprived of their civic rights, in custody, or subject to a tutorial, judiciary or administrative decision. * Patients subject to a protection measure. * Patients in a critical medical situation. * Patients with a personal situation judged by the investigator as unlikely to be compatible with optimal participation in the study, or which could constitute a risk for the patient. * Linguistic barrier or psychological profile preventing the patient from signing the consent form. * Patient still in an exclusion period following the participation in another clinical trial. * Patients having earned more than 4500€ in indemnities for participation in clinical trials during the previous 12 months, including this study.