A 57-year-old woman experiences frequent symptoms of heartburn. She reports no weight loss, diarrhea, or dysphagia symptoms. Past medical history is significant for vitamin B12 deficiency treated with monthly injections. She appears well, and the examination is normal, except for some patches on her arms where she has lost all the skin pigmentation. Her upper endoscopy is normal, except for gastric biopsies commenting on gastritis.For the above patient with gastritis, select the most likely mechanism of injury.