A 26-year-old female call-centre worker presents with a 4-month history of intensely itchy, red, scaly rash around her umbilicus that flares 24–48 hours after wearing low-rise jeans with a metal belt-buckle. She also reports similar eruptions behind her earlobe (costume jewellery earrings) and left wrist (metal wristwatch). On examination, the umbilical lesion marked **A** shows a well-demarcated, erythematous, scaly patch with pinpoint vesicles and central lichenification, precisely matching the belt-buckle contour. Dimethylglyoxime testing on the buckle produces a pink-red colour change. Patch testing with nickel sulfate 5% shows a strong positive reaction (++). Which of the following best describes the pathophysiological mechanism underlying the lesion marked **A**?