A 68-year-old Indian man presents with progressive weakness and fatigue over 3 months. He reports difficulty climbing stairs and lifting objects. On examination, his strength improves after brief rest and worsens with sustained activity. Serology is positive for anti-acetylcholine receptor (AChR) antibodies. Repetitive nerve stimulation shows a decremental response. Which of the following best explains the pathophysiology of his neuromuscular junction dysfunction?