Monocytoid B-cell lymphoma. A novel B-cell neoplasm.

Monocytoid B lymphocytes (MBLs), originally described as part of the histologic picture of toxoplasmic lymphadenitis, have been recognized as a reactive component in a variety of lymph node disorders. The authors now report 3 cases of non-Hodgkin's lymphoma in which a multidisciplinary approach allowed them to confirm the existence of a malignant lymphoma composed of the neoplastic counterpart of the MBLs found in nonneoplastic disorders. In all 3 cases, the lymphoma was composed of a relatively monomorphous infiltrate of atypical MBLs that had rather uniform-appearing nuclei and had well-defined, moderately abundant pale cytoplasm. The pattern of lymph node involvement in all 3 cases was predominantly sinusoidal and interfollicular. The neoplastic lymphoid cells were strongly positive for B-cell-restricted antigens; the light- and heavy-chain phenotypes were kappa-IgM (2 cases) and kappa-IgG (1 case). In all 3 cases, rearrangement of heavy- and/or light-chain genes was clearly identified by Southern blot hybridization. The name "monocytoid B-cell lymphoma" is proposed for this newly described malignant B-cell neoplasm.