A 45-year-old woman presents with a 2-day history of fever, pain, and swelling of the left lower leg. On examination, there is a bright red, warm, tender area with marked edema. The structure marked **B** in the diagram—the sharp, distinct border between affected and unaffected skin—is a key clinical feature that helps differentiate this condition from cellulitis. Which of the following best explains the pathophysiological basis for this sharply demarcated border in erysipelas?