B cells and plasma cells in mycosis fungoides. A study including cases with B cell follicle formation or a monotypical plasma cell component.

B cells formed a distinctive component (greater than 5%) of the infiltrates in specimens of 28% (n = 12) of the 43 cases of mycosis fungoides studied. A large majority expressed surface immunoglobulin (Ig) D and/or IgM and a small minority IgA, similar to the proportions in inflammatory infiltrates. There was a prevalence of lambda light-chain expression. In about half of these cases, specimens revealed the formation of B cell follicles at various stages of development. Plasma cells formed a substantial number (greater than 25/mm2) in specimens from 21% (n = 9) of the patients. In six of these cases, including all four cases with advanced follicle formation, the same specimens were also rich in B lymphocytes. In most of the specimens the varying majorities of IgA or IgG producing cells and the polytypical expression of kappa and lambda light chains were similar to the patterns in inflammatory disorders. Two cases showed a striking prevalence of IgG, kappa-positive plasma cells, which was present in multiple specimens of one case and was associated with atypical plasmacytoid cells in a tumor of the other case. The pertinence of these observations for a special relationship between the abnormal T cells and the B cell component is discussed.