Immunosecretory and plasma cell disorders cover a broad spectrum of clinical and pathologic entities. Some of the processes, such as monoclonal gammopathy of undetermined significance (MGUS), have relatively indolent behavior, while others, such as plasma cell leukemia, are associated with very poor prognosis and high mortality. This group of disorders includes lymphoplasmacytic lymphoma (LPL), a neoplastic entity which overlaps both with B-cell lymphoma and with the immunosecretory disorders group.
Benign plasma cells and reactive plasmacytosis
Plasma cells are a normal component of adult bone marrows. They typically represent about 0–1% of the overall cellularity seen in bone marrow aspirate smears (Foucar, 2001). Levels above 5% are considered abnormal in immunologically unstimulated marrows. In normal bone marrow biopsies, plasma cells are most typically located in perivascular locations.
In Wright–Giemsa-stained preparations, plasma cells have a distinctive, light to dark blue cytoplasm with an eccentrically placed nucleus. The nuclear chromatin is quite dense, and in appropriately thin histologic sections is classically described as “clockface chromatin.” Adjacent to the nucleus is a clearing in the cytoplasm, referred to as a hof, which represents the Golgi apparatus of the cell.
The cytologic features of benign and malignant plasma cells can overlap (Fig. 12.1). Inclusions which may be seen in plasma cells include Russell bodies, which are globular eosinophilic collections of immunoglobulin in the cytoplasm. Dutcher bodies are not true nuclear inclusions but rather cytoplasmic inclusions that overlie the nucleus. Dutcher bodies are only rarely seen in benign proliferations of plasma cells.