Table 2.

Differential diagnostic considerations in plasmacytosis.

	Diagnostic entity	Features	Immunohistochemistry/special stains
Neoplastic (monoclonal)	Marginal zone lymphoma	Plasma cells at periphery of germinal centers	CD20, CD79a, PAX5, BCL2+; BCL6, CD10−
	Plasmacytoma	Large infiltrate of monoclonal plasma cells	CD79a+; CD19, CD20−
	Leukemia cutis (plasma cell leukemia)	Infiltrates of atypical-appearing plasmacytoid cells	CD38, CD138+; CD20+/−; CD19, CD49e−
Inflammatory (polyclonal)	Castleman's disease (plasma cell variant)	Lymph nodes with hyperplastic follicles and interfollicular sheets of (lambda restricted) plasma cells	HHV-8+
	Pseudolymphoma	Circumscribed follicles of lymphocytic infiltrates with plasma cells at periphery; clinical correlation is paramount	Directed to rule out lymphoma, which is more likely BCL-6 and CD10+ outside follicle and BCL-2+ within follicle; Ki-67, more diffuse staining in reactive germinal centers than lymphoma
Infectious (polyclonal)	Borrelia (erythema migrans, acrodermatitis chronica atrophicans)	Perivascular lymphocytic infiltrate, rich in plasma cells	Wright-Giemsa, Warthin-Starry (silver stains)
	Syphilis	Dense plasma cell predominant dermal infiltrate, elongated rete ridges, and endothelial swelling	Silver stains, T. Pallidum immunohistochemistry