A 30-year-old woman has had gradually increased muscle weakness with myalgia for the past year. She now has difficulty getting up from a chair and climbing stairs. She does not have weakness in her hand muscles. Physical examination reveals a fine violaceous rash on her face, predominantly palpebral. Dusky, flat, red patches are present on her elbows, knees, and knuckles. Laboratory studies show serum creatine kinase of 620 U/L. A deltoid biopsy specimen is obtained, and on microscopic examination shows a mononuclear inflammatory cell infiltrate around small blood vessels and groups of atrophic myofibers at the periphery of fascicles. What mechanism is most likely responsible for her disease?