Erythema Nodosum — Septal Panniculitis

A 27-year-old woman presents with 10 days of fever, arthralgias of the ankles and knees, and malaise. Examination reveals tender, erythematous, warm, poorly demarcated subcutaneous nodules 2–5 cm in diameter symmetrically distributed over the anterior shins (marked **A** in the diagram). The lesions are intensely painful and have evolved from bright red to violaceous over the past week. Chest radiograph shows bilateral hilar lymphadenopathy without parenchymal infiltrates. Deep incisional biopsy through subcutaneous fat reveals septal panniculitis without vasculitis, with Miescher radial granulomas. ASO titer is normal, throat culture is negative, and Mantoux test is negative. ACE level is elevated. Which of the following best describes the pathophysiology of the lesions marked **A**?