Allergic Contact Dermatitis: Early Recognition and Diagnosis of Important Allergens

Sharon E. Jacob; Tace Steele


Dermatology Nursing. 2006;18(5):443-439, plus 4. 

In This Article

Allergic Contact Dermatitis Mechanism of Action

Allergic contact dermatitis represents a delayed-type hypersensitivity reaction (Type IV hypersensitivity). There are four types of hypersensitivity reactions, which include Type I, immediate-type-IgE-mediated reactions; Type II, antibody-mediated reactions; Type III, immune complex deposition reactions; and Type IV, T-cell-mediated delayed-type reactions.

In type IV reactions, the primary step is sensitization. This is the result of absorption of an allergen chemical into the skin which elicits an immune response that is remembered on subsequent allergen exposures. These allergens are low-molecular-weight substances that easily penetrate the stratum corneum and covalently bind to keratinocytes in the stratum spinosum below (Yunginger, 2003). This alteration on the surface of the keratinocyte allows for recognition by the antigen-presenting cells of the epidermis - Langerhans cells (Hogan, 2005). These small chemicals bound to the Langerhans cells are taken to the lymph nodes and presented to naïve CD4 T cells. A complex reaction occurs on allergen presentation and cytokines are released which promote proliferation of a clonal population of memory T cells. The memory cells with their new antigen-specific receptors return to the site of exposure and recruit more inflammatory mediators.

Within the first 24 hours after re-exposure to an allergen, the Langerhans cells present the allergen to the T cells. Within 48 hours after re-exposure, there is an overt inflammatory reaction to the allergen (Mydlarski, Katz, Mamelak, & Sauder, 2003). This dermatitis can persist for 3 to 4 weeks even after the antigen is removed (Habif, 2004). The duration and intensity of the allergic reaction depends on the patient's sensitivity to the allergen and the concentration of the allergen absorbed (Habif, 2004). Urushiol is the allergen in the sap of poison ivy, poison oak, and other plants in the Anacardiaceae family. It is such a strong sensitizer that it can produce intense inflammation in weak concentrations and can cause sensitization in 10 to 14 days after only one exposure (Habif, 2004). Although a strong allergen, there are several other reasons that poison ivy might be thought to cause immediate reactions. First, exposure can happen at any point in life, and not all patients will remember initial exposure. This first exposure could have simply been brushing by a plant in childhood. Second, urushiol is within the sap which is extremely adherent to skin. In fact after 30 minutes, only 10% of the allergen can be washed off at all (Habif, 2004). This causes the patient to absorb a high concentration of the allergen which is one of the determining factors for intensity of a reaction. Another phenomenon that occurs in allergic contact dermatitis is cross sensitization. This is a process that occurs when a patient already sensitized to one allergen becomes sensitized to a second with similar structure. This can occur through transepidermal absorption, inhalation, or ingestion of allergens (Habif, 2004). Important cross sensitizers to poison ivy are poison oak and sumac, Florida Holly, mango rind, and cashew nut resin (Habif, 2004).


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