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Answer
Answer: D) IgA
This is a classic presentation of Dermatitis Herpetiformis (DH), the skin manifestation of celiac disease.
Key clues in the stem:
- Intensely itchy papules/vesicles over elbows and knees (extensor surfaces) - the hallmark distribution
- GI symptoms (diarrhea, abdominal pain, bloating) triggered by whole wheat (gluten)
- Direct immunofluorescence (DIF) requested
Why IgA?
In Dermatitis Herpetiformis, DIF of perilesional skin shows granular deposits of IgA in the dermal papillae. This is the pathognomonic finding and the gold standard for diagnosis.
The mechanism:
- Gluten ingestion in a genetically susceptible individual (HLA-DQ2/DQ8) triggers anti-tissue transglutaminase (anti-tTG) IgA antibodies
- In DH specifically, IgA antibodies against epidermal transglutaminase (eTG/TG3) are produced
- These IgA immune complexes deposit in the dermal papillae, activating complement and neutrophil recruitment, causing the characteristic pruritic vesiculopapular eruption
Why not the others?
- IgG - seen in bullous pemphigoid (linear IgG at BMZ) and pemphigus vulgaris (IgG intercellular)
- IgM - not associated with DH
- IgD - no known role in skin disease
Treatment: Gluten-free diet (also resolves GI symptoms) + Dapsone for rapid symptom relief.
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