Plasmocytes

Plasmocytes, also known as plasma cells, are terminally differentiated B lymphocytes that synthesize and secrete antibodies. They arise from activated B cells after antigen exposure and T helper cell signals, progressing through plasmablasts and germinal-center–derived maturation. In germinal centers, B cells undergo somatic hypermutation and class-switch recombination, with some differentiating into long-lived plasma cells or memory B cells.

Anatomy and location: plasma cells are found in bone marrow and secondary lymphoid organs, and they can accumulate at sites of inflammation. Morphologically, they have an eccentrically placed nucleus with clock-face chromatin, a cytoplasm rich in rough endoplasmic reticulum, and a prominent perinuclear Golgi apparatus, reflecting their high-rate antibody production.

Function: their primary role is humoral immunity through antibody production. The antibodies neutralize pathogens, mark them for phagocytosis, and activate the complement system. Plasma cells produce immunoglobulins of various isotypes (IgG, IgA, IgM, IgE, IgD) depending on signals during B cell maturation; mucosal surfaces often favor IgA production, while systemic responses emphasize IgG. Long-lived plasma cells in the bone marrow can maintain baseline antibody levels for years, providing lasting immunity.

Clinical relevance: neoplastic plasma cell disorders include plasmacytoma and multiple myeloma, characterized by clonal expansion of malignant plasma cells and monoclonal antibody production. Other conditions involve plasmacytosis or lymphoplasmacytic neoplasms with excess immunoglobulin production. Normal plasma cell homeostasis is essential for sustained humoral defense.