Allergic contact dermatitis (ACD) is a common inflammatory skin disease, which represents a major public health issue in industrialized countries. ACD is induced by repeated contact of individuals with environmental chemicals and is characterized by a delayed type IV hypersensitivity response with skin inflammation mediated by allergen-specific T cells in sensitized individuals.

The current diagnosis is based on clinical examination, assessment of environmental exposures and patch testing. Although the robustness of patch tests has long been established, this method can sometimes give inconclusive results, leading to problems in disease management.

Preliminary results indicate that the molecular analysis of Patch-Tests (PT) reactions could allow a more reliable diagnosis. Importantly, this gene profiling approach may help to identify patients with false positive PT reactions, i.e. patients whose PT reactions did not show any "allergy signature".

However, it remains to be demonstrated that the presence or absence of allergy biomarkers in PT lesions are indeed predictive of ACD response in patients.

The main objective is to describe the correlation between these molecular signatures and the reactivity of individuals when they are exposed to allergenic compounds under conditions of use (using ROAT test).

Inclusion Criteria:

• Patient, male or female, over 18 years of age.
• Patient with at least one positive/doubtful patch test reaction for nickel, limonene hydroperoxide and/or linalool hydroperoxide
• Patient agreeing to undergo skin biopsies and blood sampling
• Patient agreeing to non-identifying pictures being taken of lesions
• Patient available to carry out skin tests and their interpretation
• Patient affiliated to or benefiting from a social security regime
• Patient having been informed and having signed a written, free and informed consent.

Exclusion Criteria:

• Patient with active dermatitis lesions on the forearm
• Patient with a history of allergic reaction to a local anesthetic product
• Patient with wound healing disorders (hypertrophic or keloids scars)
• Patient with hematological disorders
• Patient having topical treatments with corticosteroids or immunomodulators on the forearms during the 21 days prior to the start of the study
• Patient having had excessive exposure to ultraviolet during the 21 days prior to the start of the study.
• Patient on systemic corticosteroid therapy, immunosuppressants or biological therapy.
• Patient whose follow-up is impossible for reasons psychological or geographical.
• Patient taking part in another clinical study
• Protected patient: adult under guardianship, curatorship or other legal protection, deprived of liberty by judicial or administrative decision
• Pregnant, breast-feeding or parturient woman