Progressive multifocal leukoencephalopathy associated with immune dysregulation

B-cell lymphopenia, polyclonal hypergammaglobulinaemia and raised free light chains, suggesting functional immunodeficiency despite numerically preserved T-cell subsets.This case is educationally important as it highlights the diagnostic challenge of recognising PML in patients with apparently preserved T-cell counts, the value of extended immune profiling (B-cell enumeration and immunoglobulin characterisation) in atypical presentations and the therapeutic dilemmas that arise when balancing immune recovery against control of autoimmune disease.