Pemphigoid Gestationis

A 28-year-old G2P1 at 32 weeks gestation presents with severe pruritus and a 2-week eruption of urticarial plaques that have progressed to tense bullae. The rash began in the periumbilical region and spread centrifugally to the abdomen, thighs, and extremities, sparing the palms, soles, and mucous membranes. Direct immunofluorescence of perilesional skin shows linear deposition of C3 and IgG along the basement membrane zone. The condition marked **B** in the diagram is suspected. Which of the following best describes the pathogenic mechanism underlying this autoimmune blistering disease?