Plasmacytoid lymphocytes are atypical, reactive lymphocytes often seen in severe (viral) infections such as those with Epstein-Barr Virus. These large, predominantly round cells have a large nucleus with a coarse chromatin pattern and deep dark blue cytoplasm. A white brightening may be seen next to the nucleus (perinuclear court). The nucleus is usually bean-shaped.

It is important to distinguish between plasma cells and plasmacytoid cells, because prominent presence of plasmacytoid cells is relatively harmless, whereas large numbers of plasma cells are indicative of a malignancy.