Review Question - QID 109137

In a patient with localized, itchy, scaly plaques of the abdomen, an allergic contact dermatitis should be suspected (e.g., nickel). This is caused by T-cell-mediated cellular damage (type IV hypersensitivity reaction).

Type IV hypersensitivity reactions begin with sensitization of T-cells to a specific antigen. Subsequent exposure to the antigen results in activation of effector T-cells and macrophages that lead to cell-mediated tissue damage. Examples of type IV hypersensitivity reactions include allergic contact dermatitis to metals such as nickel, exposure to poison ivy, graft-versus-host-disease, and chronic transplant rejection. In patients with localized itchy dermatitis in areas with potential contact to metals (e.g., earlobes from earrings, wrist from wristwatches, and abdomen from belt buckles), an allergic contact reaction to metals such as nickel should be suspected.

Figure A shows a well-circumscribed, scaly, plaque on the lower abdomen near the area where a metal belt buckle may contact the skin.

Incorrect Answers:
Answer 1: IgE-mediated (type I) hypersensitivity reactions are characterized by mast cell and basophil release of histamine and other vasoactive compounds after exposure to an allergen. Examples of type I hypersensitivity reactions include allergic rhinoconjunctivitis, asthma, and anaphylaxis.

Answer 2: Immune-complex-mediated (type III) hypersensitivity reactions are characterized by deposition of antigen-antibody immune complexes that result in fixation and activation of complement. Examples include serum sickness, systemic lupus erythematosus, and rheumatoid arthritis.

Answer 4: Fungal infections such as tinea corporis can also present with circular, scaly patches, but typically have a central clearing and an edge of advancing scale. The anatomic location of the rash in this patient (in an area of likely contact to metals) further makes allergic contact dermatitis more likely.

Answer 5: Deficiency of filaggrin, an epidermal protein that acts to link keratin fibers in epithelial cells, is associated with disorders of dry skin including atopic dermatitis. Atopic dermatitis can present with itchy, scaly plaques but are typically localized in the flexural areas in adults.

Bullet Summary:
Type IV hypersensitivity reactions such as allergic contact dermatitis are characterized by delayed, T-cell-mediated reactions and include allergic contact dermatitis.