A 28-year-old man presents with fever, urticaria, and polyarthralgia 7 days after receiving an intramuscular injection of tetanus antitoxin (horse serum). Skin biopsy shows leukocytoclastic vasculitis with fibrinoid necrosis of small blood vessels. The pathologic mechanism marked **C** in the diagram is responsible for this clinical presentation. Which of the following best describes the sequence of events in this hypersensitivity reaction?