Mantle cell lymphoma presenting as bullous pemphigoid: a rare paraneoplastic syndrome.

A 60-year-old man presented with severe, treatment-refractory bullous pemphigoid and was subsequently diagnosed with stage IV pleomorphic variant mantle cell lymphoma (MCL). Direct immunofluorescence and histopathology confirmed bullous pemphigoid, while peripheral blood smear abnormalities and cross-sectional imaging prompted further workup. Lymph node biopsy demonstrated a monotypic CD20+, PAX5+, CD5+, cyclin D1+B cell lymphoproliferative disorder, with CCND1::IGH fusion confirmed by fluorescence in situ hybridisation. The temporal association between cutaneous disease onset and lymphoma diagnosis, together with improvement of bullous pemphigoid following MCL-directed therapy, supports a paraneoplastic relationship. Bullous pemphigoid is most commonly idiopathic but is recognised in association with haematological malignancies. Its occurrence with MCL is exceedingly rare, with only two prior cases in the literature. This case reinforces the importance of screening for occult malignancy in patients with atypical, severe or refractory bullous disease.