❑ About 30% of occupational skin disorders are allergic contact dermatitis.
❑ In sensitized persons, inflammation begins about 12 hours after exposure to an allergen.
Although contact allergens produce sensitization in only a small percentage of exposed persons, allergic contact dermatitis constitutes about 30% of the skin disorders found in the workplace. Once a person has been sensitized to an offending substance, further exposure may result in relatively rapid development of local inflammation with erythema, papule formation, induration, and weeping vesiculation. Inflammation usually begins about 12 hours after exposure; intensity peaks in 50 hours or more. The rash may spread locally around the margins of the original site or to distant sites that did not have contact with the allergen. Potentially, the entire skin surface could become involved (a condition known as erythroderma or exfoliative dermatitis).
❑ Cross-reactivity with antigenically similar substances can occur with allergic contact dermatitis.
❑ The clinical and histologic appearances of allergic and irritant contact dermatitis are similar.
Allergic contact dermatitis results from a true allergic (i.e., cell-mediated) sensitization to the offending substance. Cross-reactivity with antigenically similar substances may occur. Initially, during the refractory period, the patient may be exposed without developing a reaction. During the induction phase, which may last from 4 days to several weeks (usually about 14 to 21 days), the development of complete allergic sensitization occurs as the allergen comes in contact with the skin. After the skin is fully sensitized, further contact with the allergen may result in rapid and severe dermal manifestations. When no further contact with the allergen occurs, the patient is in the period of persistence of sensitivity. The level of sensitivity can decrease over time, but sensitization may be lifelong.
Most allergens that cause allergic contact dermatitis have molecular weights of less than 500 daltons. The allergens are haptens rather than complete antigens; they must penetrate the skin and combine with endogenous proteins to form full antigens. Langerhans cells play a key role in then presenting the antigen to T lymphocytes, thereby activating the T cells. The sensitized T cells proliferate in the paracortical regions of the lymph nodes and produce effector and memory lymphocytes that remain in the general circulation. On subsequent contact with the complete antigen, the effector cells release lymphokines that may result in rapid and severe, local inflammation.
Many factors can affect the development of allergic contact dermatitis, including characteristics of the allergen itself, patient factors, and environmental conditions. Allergen factors include the physiochemical nature of the allergen (e.g., lipophilicity, solubility, and inherent sensitizing potency), concentration, total dose that comes in contact with the skin, anatomic site of contact, number and frequency of exposures, and occlusion by clothing or gloves.
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