A 28-year-old male smoker presents with a 2-week history of hemoptysis, dyspnea, and hematuria. Serum creatinine is 3.2 mg/dL with active urinary sediment showing RBC casts. Renal biopsy demonstrates crescentic glomerulonephritis with linear IgG deposition along the glomerular basement membrane on immunofluorescence. The structure marked **B** in the diagram is identified as the pathogenic agent. Which of the following best describes the molecular target of this antibody?