Plasma cells are terminally differentiated B-lymphocytes that have developed a characteristic morphology while actively producing and releasing immunoglobulins. While plasma cells have their origins in the bone marrow as B-cells, they usually leave the bone marrow to develop and mature in the lymph nodes or spleen. Plasma cells begin to produce immunoglobulins after being stimulated by T-cells and exposed to processed antigens.
Under normal circumstances, plasma cells are not a large percentage of the lymphoid cells found in a marrow. They are usually placed in a separate category in the differential, unlike viral/atypical lymphs. There can be a relative increase in plasma cells in reactive marrows, and both plasma cells and their early precursors will be markedly increased in plasma cell disorders.
While mature plasma cells somewhat resemble lymphocytes, there are a few important differences. The size of the cell is usually larger with more abundant cytoplasm. The nucleus is eccentrically placed and the overall shape of the cell generally resembles a wedge or comet with the nucleus leading the cytoplasm. The chromatin is just as thick and clumpy as a lymphocyte's but is aligned in a more "spokey" or "clockwork" pattern. The cytoplasm is usually more basophilic than the cytoplasm of a normal lymphocyte and will have a well-defined perinuclear halo or noticeable clearing in the golgi area. Vacuoles may or may not be present.
Notice the size of the single plasma cell in the top image (see red arrow). It is larger than the neutrophil precursors surrounding it and is almost rectangular in shape. Observe that the nucleus leads the cytoplasm, causing the wedge or comet shape. Notice the prominent perinuclear halo.
Find the two plasma cells in the upper left corner of the second image. There is much more cytoplasm in these plasma cells compared to the occasional lymphocyte present in the field. Notice the eccentric nuclear placement as well as the characteristic clearing in the golgi area.