Multiple aspects of contact allergy: immunology, patch test methodology and epidemiology

Abstract

Contact allergy is a type IV hypersensitivity reaction that presents as an itchy skin rash, is frequent, and impairs quality of life. Patch testing diagnoses specific contact allergies by exposing individuals to a large range of contact allergens and forms the basis of epidemiological data. Epidemiological studies help identify groups at risk for developing contact allergy overall and to specific contact allergens. One common contact allergen is p-phenylenediamine, a chemical with good colouring properties present in both permanent hair dyes and henna tattoo, among other products. As a means of primary prevention of contact allergy to hair dyes, a less sensitizing chemical, 2-methoxyethoxy-p-phenylenediamine, has been developed. We found that for individuals already sensitized to p-phenylenediamine, 2-methoxyethoxy-p-phenylenediamine can’t be considered a safe alternative, although the threshold for elicitation was lower than for p-phenylenediamine. Patch testing, the diagnostic test for contact allergy, is performed often once, and as contact allergies are considered permanent, diagnosis of a contact allergy often necessitates avoidance for life. We found however that the persistence of positive reactions was only 67% in patients who were tested twice. Many individuals have more than one contact allergy. The prevalence of polysensitisation - three or more contact allergies to unrelated contact allergens - was found to be about 7% in patients included in a large European cohort. The majority were female and over the age of 40, implying (cumulative) exposure to be the likely explanation of polysensitisation.