Plasma cell: A type of white blood cell that produces and secretes antibodies. A plasma cell is a fully differentiated, mature lymphocyte in the B cell lineage. As with most cell types, plasma cells can mutate to give rise to cancer. Plasma cell malignancies include plasmacytoma, multiple myeloma, Waldenstrom macroglobulinemia, and plasma cell leukemia. Also known as plasmacyte.