Contact Dermatitis: Allergic vs Irritant

Overview / Definition Contact dermatitis is a common inflammatory skin condition that occurs when the skin comes into contact with a substance that causes an adverse reaction. It can be categorized into allergic contact dermatitis (ACD), which results from an immunologic response to an allergen, and irritant contact dermatitis (ICD), which arises from direct damage to the skin barrier by irritants. Epidemiology Contact dermatitis is one of the most prevalent skin conditions encountered in dermatology. The prevalence of ACD is approximately 15-20% in the general population, while ICD is even more common, accounting for up to 80% of all cases of contact dermatitis. ACD is more prevalent in individuals with a history of atopy. ICD can affect anyone, particularly those with occupational exposure to irritants. Pathophysiology / Mechanism The pathophysiology of contact dermatitis differs between its two forms: Allergic Contact Dermatitis (ACD): This is mediated by a Type IV hypersensitivity reaction, where prior sensitization to an allergen leads to a delayed immune response upon re-exposure. Irritant Contact Dermatitis (ICD): This involves direct damage to the skin’s stratum corneum by chemical agents, resulting in an inflammatory response without a prior sensitization phase. Clinical Presentation The clinical features of contact dermatitis can vary significantly between ACD and ICD: Allergic Contact Dermatitis: Typically presents as pruritic erythematous papules and vesicles on exposed areas of skin. May exhibit linear or well-defined borders, often following the pattern of contact with the allergen. Common allergens include nickel, poison ivy, fragrances, and preservatives. Irritant Contact Dermatitis: Characterized by dryness, redness, and scaling, often with fissures and crusting. Common irritants include soaps, detergents, and solvents. Typically occurs at sites of frequent exposure. Diagnosis / Workup The diagnosis of contact dermatitis is primarily clinical but ma