human mucosa or mucosa-associated lymphoid tissues (MALT). Originally it was thought that the continuous production of antibodies is a result of constant replenishment of short-lived plasma cells by memory B cell re-stimulation. Recent findings, however, show that some PC are truly long-lived. The absence of antigens and the depletion of B cells does not appear to have an effect on the production of high-affinity antibodies by the LLPC. Prolonged depletion of B cells (with anti-CD20 monoclonal antibody treatment that affects B cells but not PC) also did not affect antibody titres. LLPC secrete high levels of IgG independently of B cells. LLPC in bone marrow are the main source of circulating IgG in humans. Even though IgA production is traditionally associated with mucosal sites, some plasma cells in bone marrow also produce IgA. LLPC in bone marrow have been observed producing IgM. Clinical significance Plasmacytoma, multiple myeloma, Waldenström macroglobulinemia, heavy chain disease, and plasma cell leukemia are malignant neoplasms ("cancer") of the plasma cells. Multiple myeloma is frequently identified because malignant plasma cells continue producing an antibody, which can be detected as a paraprotein. Common variable immunodeficiency is thought to be due to a problem in the differentiation from lymphocytes to plasma cells. The result is a low serum antibody level and risk of infections. Primary amyloidosis (AL) is caused by the deposition of excess immunoglobulin light chains which are secreted from plasma cells. See also Leukocyte Plasma cell dyscrasia References External links Histology at wadsworth.org B cells Connective tissue cells Human cells Immune system Immunology Lymphocytes