Myeloma is a cancer of the plasma cells in the bone marrow, the spongy tissue inside of bones. Plasma cells are a part of the body's immune system and produce antibodies that help the body fight infection. Abnormal plasma cells can suppress the growth of other cells in the bone that produce red blood cells, white blood cells and platelets. This suppression may result in anemia (from a shortage of red blood cells), excessive bleeding from cuts (from a shortage of platelets) and a decreased ability to fight infection (from a shortage of white blood cells). Myeloma often causes structural bone damage resulting in painful fractures. Like regular plasma cells, myeloma cells can produce antibodies. However, as the myeloma cells grow uncontrollably, there is overproduction of antibodies, leading to an accumulation in the blood and urine that may cause kidney and other organ damage.

Myeloma is often called multiple myeloma because most people (90 percent) have multiple bone lesions at the time it is diagnosed. Solitary plasmacytoma is a mass of myeloma cells that involve only one site in the bone or other organs (most commonly the upper respiratory tract, including the nose and throat). Extramedullary plasmacytoma describes myeloma that started outside of the bone marrow, such as the lymph glands, sinuses, throat, liver or under the skin.