A MALIGNANT disorder of PLASMA cells, derived from B-lymphocytes (see LYMPHOCYTE). In most patients the BONE MARROW is heavily infiltrated with atypical, monoclonal plasma cells, which gradually replace the normal cell lines, inducing ANAEMIA, LEUCOPENIA, and THROMBOCYTOPENIA. Bone absorption occurs, producing diffuse osteoporosis (see under BONE, DISORDERS OF). In some cases only part of the immunoglobulin molecule is produced by the tumour cells, appearing in the urine as Bence Jones Protein.sease is rare under the age of 30, frequency increasing with age to peak between 60 and 70 years. There may be a long pre-clinical phase, sometimes as long as 25 years. When symptoms do occur, they tend to reflect bone involvement, reduced immune function, renal failure, anaemia or hyperviscosity of the blood. Vertebral collapse is common, with nerve root pressure and reduced stature. The disease is eventually fatal, infection being a common cause of death. Local skeletal problems should be treated with RADIOTHERAPY and the general disease with CHEMOTHERAPY chiefly the ALKYLATING AGENTS melphalan or cyclophosphamide. Red-blood-cell TRANSFUSION is usually required, together with plasmapheresis (see PLASMA EXCHANGE), and orthopaedic surgery may be necessary following fractures.