Increased CD5‐positive polyclonal B cells in Castleman disease: a diagnostic pitfall

Sir: CD5 B lymphocytes (B1 lymphocytes in the mouse) comprise a large proportion of B cells during early fetal life, but decrease with age. They produce polyreactive antibodies and have been implicated in autoimmune disease, including rheumatoid arthritis, Sjogren’s syndrome and systemic lupus erythematosus. Castleman disease has also been associated with autoimmune disorders. CD5 expression on B cells of the expanded mantle zones in Castleman disease has been reported in sporadic cases, and was mainly associated with the plasma cell var iant. However, most of these reports are from the older literature, and employed frozen section immunohistochemistry. Additionally, in some of the reported cases molecular studies identified clonal immunoglobulin gene rearrangements, raising the possibility of involvement by a CD5 B cell lymphoma. With the recognition that patients with monoclonal B lymphocytosis (MBL) can have limited lymph node involvement, ruling out early involvement by MBL or chronic lymphocytic leukaemia becomes of paramount importance. Similarly, CD5 B cells in the in situ form of mantle cell lymphoma can be confined to the lymphoid cuff.