Contact sensitization to hair care allergens in scalp seborrheic dermatitis: associations with disease severity and microbiota profiles.

Background: Scalp seborrheic dermatitis (SSD) is a chronic inflammatory skin disorder characterized by impaired barrier function and intolerance to topical products. However, the relationship between contact sensitization and scalp microbiota in SSD remains unclear. Methods: A total of 63 participants underwent patch testing with 62 allergens and were grouped according to the presence or absence of scalp involvement. Clinical assessments included symptom severity, transepidermal water loss, and stratum corneum hydration. Bacterial 16S rRNA V3-V4 sequencing and fungal ITS1 sequencing were performed in a subset of 36 patients with SSD to evaluate associations between allergen sensitization and scalp microbiota. Results: The most frequent sensitizers in patients with SSD were cobalt chloride, cetrimonium bromide, p-methylaminophenol, nickel sulfate, decyl glucoside, and minoxidil, although overall sensitization rates did not differ significantly between SSD and control groups. Specific allergens were associated with age, sex, disease duration, and disease severity. Increased transepidermal water loss was correlated with fragrance and preservative allergens. Minoxidil sensitization was negatively associated with Conclusion: Patients with SSD showed distinct sensitization patterns associated with clinical characteristics, barrier dysfunction, and scalp microbiota alterations. These findings suggest that patch test reactivity in SSD should be interpreted in the broader context of allergen exposure, skin barrier status, and microbial imbalance.