Follicular lymphoma with abundant PAS-positive extracellular material. Immunohistochemical and ultrastructural observations.

A finding of large amounts of extracellular amorphous material is rare in malignant lymphomas. We report four such cases of follicular lymphoma that were investigated by electron microscopy and immunohistochemistry using monoclonal antibodies. This extracellular material was clearly different from the sclero-hyalinosis commonly found in malignant lymphomas. Our observations revealed that the material is ultrastructurally composed of membranous structures, membrane-bound vesicles, and electron-dense bodies. Immunohistochemistry confirmed its neoplastic cell origin and its identity with cell membrane antigens (Dako-LC+, anti-B antibodies positive). In one case investigated using frozen sections, it showed the same immunophenotype as the neoplastic cells: SB4+ (CD19), SB2+ (CD21), TO15+ (CD22), SB3+ (CD37), IgML+, and Calla+. The mechanism for accumulation of this extracellular amorphous material is probably a function of a deregulation of cell wall synthesis and an exocytosis of cell membranes.