Differentiation between malignant B-cell lymphomas and pseudolymphomas of the skin.

Among cutaneous pseudolymphomas (PL) in a strict sense, in which the etiology is largely unknown, B-cell types, simulating malignant B-cell lymphoma, and T-cell types, simulating malignant T-cell lymphoma of the skin, can be differentiated. Criteria favoring a pseudolymphomatous rather than a malignant lymphoid infiltrate of B-cell type may be clinical (solitary lesion on the head), histologic (wedge-shaped infiltrate with follicle formation and sharply demarcated germinal centers), or cytomorphologic (polymorphous infiltrate showing an admixture of great numbers of macrophages, plasma cells, and eosinophils). Additional information can be provided by enzymophenotyping of the cells, showing the typical starry-sky pattern of the macrophages in pseudolymphomatous infiltrates, and by the demonstration of the polyclonality of the lymphoid cells in pseudolymphomas with respect to surface markers and intracytoplasmatic Ig production, as shown by immunophenotyping techniques.