Reactive plasmacytoses are expansions of plasmablasts retaining the capacity to differentiate into plasma cells.

Abstract Circulating plasma cells in 10 cases of reactive plasmacytosis had a shared phenotype with early plasma cell (CD19+CD38+ CD138+ CD40+CD45+ CD11a+ CD49e−CD56−). In most cases, a minor subpopulation of CD28+ plasma cells was also detected. Reactive plasma cells were highly proliferative, suggesting the presence of circulating progenitors (plasmablasts). After CD138+ plasma cell removal, highly proliferative CD138− plasmablasts differentiated into CD138+ plasma cells within a few days. This differentiation, which was associated with increased CD38 and decreased HLA-DR expression, was further confirmed by a large increase in intracellular Ig content (associated with Ig secretion) and w…