A 41-year-old forestry worker returns from a hiking trip in the northeastern United States and presents 12 days later with a slowly expanding rash on his left thigh, low-grade fever, headache, and myalgias. He recalls removing an engorged tick from that thigh two weeks earlier. Examination reveals an oval, erythematous, annular plaque 14 cm in diameter with a raised, warm, advancing red border. The structure marked **B** in the diagram—the central clearing producing the classic bull's-eye appearance—is present. Which of the following best describes the pathophysiology underlying this central clearing in erythema chronicum migrans?