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?
A. Aberrant expression of placental BP180 in MHC class II molecules of paternal HLA-DR3/DR4 origin, breaking maternal tolerance and triggering IgG1 autoantibodies against the NC16A domain of BP180
B. Molecular mimicry between herpes simplex virus envelope glycoproteins and epidermal basement membrane antigens, leading to cross-reactive antibody formation
C. Deficiency of regulatory T cells in pregnancy causing loss of tolerance to desmoglein 3, resulting in circulating IgG autoantibodies
D. Activation of complement-mediated destruction of hemidesmosomes through antibodies against α6β4 integrin, independent of HLA-DR presentation
Explanation
Why Option 1 is correct
The condition marked B (pemphigoid gestationis) is a rare autoimmune blistering disease of pregnancy with a well-established pathogenic mechanism. The disease arises from aberrant expression of placental BP180 (BPAG2/collagen XVII) in MHC class II molecules of paternal HLA-DR3/DR4 origin, which breaks maternal immune tolerance. This triggers the production of IgG1 autoantibodies (the "HG factor") that recognize the NC16A domain of BP180 at the basement membrane zone. Direct immunofluorescence showing linear C3 and IgG deposition along the basement membrane zone is the diagnostic gold standard and confirms the autoimmune pathogenesis. This mechanism explains the typical presentation in the second/third trimester and the characteristic flare at delivery (Bolognia Dermatology 5e; Williams Obstetrics 26e).
Why each distractor is wrong
Option 2: While the disease was historically called "herpes gestationis," this is a misnomer. Pemphigoid gestationis is NOT viral and NOT caused by herpes simplex virus. Molecular mimicry with HSV is not the pathogenic mechanism.
Option 3: Although pregnancy involves immune tolerance mechanisms, pemphigoid gestationis is not primarily a regulatory T cell deficiency disease. The pathogenesis is specific to BP180 expression in placental MHC class II molecules, not a generalized loss of tolerance to desmoglein 3 (which is the antigen in pemphigus vulgaris, a different autoimmune blistering disease).
Option 4: While complement activation (C3 deposition) is seen on direct immunofluorescence, the primary pathogenic mechanism is not direct complement-mediated destruction of hemidesmosomes via α6β4 integrin antibodies. The disease is driven by IgG1 autoantibodies against BP180, and HLA-DR presentation is essential to the mechanism.
High-YieldNEET PG
Pemphigoid gestationis = IgG1 anti-BP180 autoantibodies triggered by placental BP180 expression in paternal HLA-DR3/DR4 molecules; linear C3/IgG at BMZ on DIF is diagnostic gold standard; umbilical onset distinguishes it from PUPPP (which starts in striae).
Bolognia Dermatology 5e; Williams Obstetrics 26e
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