Piringer’s lymphadenopathy. Morphological and immunophenotypical features of monocytoid B cells in comparison with other B-cell populations. (a and b) Giemsa staining. (c) CD20 immunostaining: mantle cells and germinal center B cells are less intensively stained than the monocytoid B cells and display a slightly different red tone. (d) IgM immunostaining: monocytoid B cells are largely negative for IgM, which is different from the strongly IgM-positive follicle mantle cells. (e) IgD immunostaining. Only some monocytoid B cells are positive for IgD. Arrows show small clusters of epithelioid cells characteristic of Piringer’s lymphadenopathy. MCB, monocytoid B cells; FM, follicle mantle; GC, germinal center.