A 45-year-old man complains of recurrent nausea and abdominal distress. A gastric biopsy shows atrophy of the mucosa in the fundus and body, with marked lymphocytic infiltration. Blood studies show severe macrocytic anemia with a mean corpuscular volume of 110 mm3. In addition, numerous hypersegmented neutrophils are appreciated on a blood smear. Which of the following is the MOST likely pathogenetic mechanism of this patient's anemia?