In a case of mucous membrane pemphigoid (cicatricial pemphigoid), which structure in the basement membrane zone is primarily targeted by autoantibodies, causing a sub-basilar split?

Mucous membrane pemphigoid (MMP/cicatricial pemphigoid) is an autoimmune vesiculobullous disorder in which IgG and IgA autoantibodies target components of the hemidesmosome at the basement membrane zone (BMZ). The primary antigens include: BP180 (type XVII collagen/BPAG2) and BP230 (BPAG1), which are structural components of hemidesmosomes. This autoimmune attack causes a sub-epithelial (sub-basilar) split — separation occurs below the basal cell layer at the level of the lamina lucida. This distinguishes MMP from pemphigus vulgaris, which features an intra-epithelial split (targeting desmosomes). Hemidesmosomes attach basal keratinocytes to the basement membrane.