plasma cell

Plasma cells arise from activated B lymphocytes following antigen recognition and, typically, T-helper cell interaction (class switching and affinity maturation occur in germinal centers). They lose surface immunoglobulin expression (except for a small amount) and CD20, and instead express CD38 and CD138 (syndecan-1), which are used as immunohistochemical markers. Morphologically, plasma cells show an eccentrically placed nucleus with heterochromatin arranged in a clock-face pattern, abundant basophilic cytoplasm (due to extensive RER for immunoglobulin synthesis), and a perinuclear clear zone corresponding to a prominent Golgi apparatus used for antibody packaging and secretion.

Functionally, plasma cells are the primary effector cells of humoral immunity, secreting large quantities of antigen-specific antibodies. Long-lived plasma cells home to the bone marrow and can persist for years, providing sustained antibody titers (memory) even without repeat antigen exposure.

Clinical significance: Plasma cells are central to several important pathologies. Multiple myeloma is a malignant proliferation of plasma cells producing a monoclonal immunoglobulin (M protein), leading to lytic bone lesions, hypercalcemia, renal insufficiency (light chain cast nephropathy), and anemia (CRAB criteria). Reactive plasmacytosis can occur in chronic infections (e.g., syphilis, hepatitis) or autoimmune conditions. Plasma cells are also seen in chronic inflammation (e.g., Hashimoto thyroiditis, chronic gastritis with H. pylori, chronic endometritis). Waldenström macroglobulinemia involves lymphoplasmacytic cells secreting IgM, causing hyperviscosity syndrome. Plasma cell markers (CD138, CD38) are used diagnostically in flow cytometry and immunohistochemistry to identify plasma cell neoplasms.