dermatologic disorders
Contact Dermatitis
Pathophysiology of Contact Dermatitis
Contact dermatitis is caused by irritants or allergens. ICD is a nonspecific inflammatory reaction to toxic substances contacting the skin. Numerous substances can irritate the skin, including Chemicals (eg, acids, alkalis, solvents, metal salts) Soaps (eg, abrasives, detergents) Plants (eg, poinsettias, peppers) Chronic moisture (eg, from body fluids, urine, and saliva) Properties of the irritant (eg, extreme pH, solubility in the lipid film on skin), environment (eg, low humidity, high temperature, high friction), and patient (eg, very young or old) influence the likelihood of developing ICD. ICD often can be divided into categories: Acute ICD: Potent irritants, such as caustic chemicals, can damage the skin immediately, typically manifesting with acute burning or stinging pain. Chronic or cumulative ICD: Less potent irritants require longer (chronic) or repeated (cumulative) periods of skin contact to cause ICD; these forms typically manifest with pruritus. Occupational ICDis ICD caused by one or more of the many possible work-related skin irritants. It can be acute, chronic, or cumulative. Atopic disorders increase risk of ICD because of impaired skin barrier function and lower threshold for skin irritation. ( see Chemical photosensitivity ) is a variant in which a topical (eg, perfumes, ACD is a type IV, T-cell–mediated, delayed-type hypersensitivity reaction to an environmental allergen that has 2 phases: Sensitization to an antigen Allergic response after reexposure In the sensitization phase, allergens are captured by Langerhans cells (dendritic epidermal cells). When activated by innate immunity cascades, these cells migrate to regional lymph nodes, where they process and present the antigen to naive, antigen-specific T cells. When a naive T cell recognizes its antigen via binding to its T-cell receptor, it expands clonally and differentiates into memory/effector T cells. The sensitization phase, which is asymptomatic, may be brief (6 to 10 days for strong sensitizers such as poison ivy) or prolonged (years for weak sensitizers such as sunscreens, fragrances, and glucocorticoids). During differentiation, sensitized T cells become able to express cutaneous homing antigens (eg, cutaneous lymphocyte antigens) that enable them to migrate from cutaneous capillaries to the epidermis. When antigen-presenting cells present the antigen to the sensitized T cells, the T cells can expand and trigger an inflammatory reaction at that location (elicitation phase of ACD), resulting in the characteristic symptoms and signs of ACD. Multiple allergens can cause ACD (see table Causes of Allergic Contact Dermatitis ). Nickel sulfate is the most common contact allergen in most populations. The Photoallergic contact dermatitisis a variant of ACD, in which a substance becomes sensitizing only after it undergoes structural change triggered by ultraviolet light. Reactions may extend to non–sun-exposed skin. Typical causes include fragrances (eg, musk ambrette, sandalwood), nonsteroidal anti-inflammatory drugs, and sunscreen filters. 