A 14-year-old, 6 years post-heart transplant for dilated cardiomyopathy, presents for routine follow-up. He has been intermittently non-compliant with his medications. An endomyocardial biopsy shows histologic damage, immunostaining positive for the complement component C4d, and macrophage expression of CD68, with minimal T-cell infiltration. Which of the following is the most accurate description of this process?