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.