Nodular B-Lymphocyte Reactive Patterns: Reactive Nodular B-Cell Pattern

Abstract

This chapter focuses on nodular B-lymphocyte reactive hyperplasia (pseudolymphoma) including classification, historical overview and clarification of nomenclature, epidemiology, clinical and histomorphological features, cytogenetics and molecular features, and clinical course. At the outset, we give an overview of normal B-lymphocyte ontogeny. Entities specifically discussed in-depth include idiopathic lymphocytoma cutis, lymphomatoid drug-induced pseudo B-cell lymphoma, and cutaneous lymphoid hyperplasia secondary to foreign bodies – reaction to tattoo, pseudo B-cell lymphoma secondary to infections, pseudolymphoma at sites of vaccination, persistent nodular arthropod-bite reactions, and nodular scabies. We conclude by discussing mimickers, the significance of clonality and clues to diagnosis. There is no single histopathologic criterion to cutaneous B-cell pseudolymphomas (CBPL) from cutaneous B-cell lymphomas (CBCL). Histologic diagnosis of CBPL depends on two considerations: (1) the architecture of the infiltrate and (2) the composition and cytomorphology of cells in the infiltrate. Histopathologic features that favor CBPL over CBCL include (1) acanthosis, (2) a top-heavy infiltrate, (3) a mixed (T- and B-lymphocyte-rich) infiltrate, (4) absence of mitosis outside of the germinal center and/or necrosis, (5) regular appearing germinal centers, (6) presence of tingible bodies in the germinal centers, (7) lack of cohesion of lymphoid cells, (8) infiltrative border (concave), (9) preservation of adnexal structures with the infiltrate respecting adnexal epithelium, and (10) stromal fibrosis. A useful immunohistochemical feature for distinguishing cutaneous follicle-center cell lymphoma from cutaneous lymphoid hyperplasia is the presence of small clusters of CD10+/Bcl6+ lymphocytes in the interfollicular zones and positive Bcl2 staining in the follicles in the former. Furthermore, there is a low MIB-1 proliferation fraction in lymphoma compared with cutaneous lymphoid hyperplasia.