In multiple myeloma, cancerous plasma cells are found in the bone marrow. The bone marrow is the spongy tissue inside the large bones in the body. The plasma cells also may cause the bone to break down. The plasma cells can collect in the bone to make small tumors called plasmacytomas. Plasma cells are found in bone marrow, the blood-producing tissue that fills in spaces within bone.
Plasma cells produce antibodies, substances that help the body fight infection.
Usually, plasma cells make up about one to two percent of all cells in bone marrow. In myeloma, a plasma cell becomes malignant. It grows continuously especially in the marrow destroying normal bone tissue, causing pain, and crowding out normal blood cell production. Malignant plasma cells produce an abnormal protein called monoclonal immunoglobulin. The onset of myeloma interferes with normal production of antibodies and makes myeloma patients susceptible to infections.