This drawing illustrates the microscopy of an oral bulla in MMP. The autoantibodies destroy the union between connective tissue and epithelium with consequent accumulation of plasma and cell debris below the epithelium and formation of a vesicle or a bulla (a vesicle measures up to 5 mm in diameter, a bulla is a large vesicle). The epithelium is completely detached from the connective tissue. Hence, the cavity of the vesicle is entirely infraepithelial. This is a basic difference with pemphigus vulgaris, where the vesicle is intraepithelial. The lumen of the vesicle is occupied by serum and cell debris.