A young woman developed red, itchy papules on her wrist 2 days after wearing a new bracelet. This reaction was caused by

• A. IgE-sensitized mast cells in the skin.
• B. antigen-antibody complexes in the skin.
• C. damage to the skin cells by antibodies and complement.
• D. an inflammatory response induced by cytokines released from Th1 cells.

Answer: (D)

This is a delayed-type hypersensitivity reaction, a T cell–mediated skin allergy to a substance in the bracelet. When the skin is exposed, small molecules from the bracelet can bind to skin proteins, forming a hapten-protein complex. Langerhans cells present this complex to T helper cells in nearby lymph nodes, sensitizing them. Upon re-exposure, memory Th1 cells release cytokines such as IFN-γ and other inflammatory mediators, recruiting macrophages and producing the local inflammation that shows up as red, itchy papules after a day or two. This pattern fits a T cell–driven, cytokine-mediated response rather than an immediate IgE-mediated reaction, immune complex–mediated damage, or antibody-dependent cytotoxic processes.