In multiple myeloma patients, mutated plasma cells - otherwise known as myeloma cells - grow unregulated by the processes that normally control cell division and death. These myeloma cells then travel through the bloodstream and collect in the bone marrow.

Myeloma cells damage other cells in the bone that produce white and red blood cells and platelets, often causing anemia and a decreased immune function. The overgrowth of plasma cells in the bone marrow often leads to structural bone damage, resulting in bone pain and fractures. Myeloma cells also produce abnormal antibodies that cannot effectively fight infection.

As tumors grow, they invade the hard, outer part of the bone. In most cases, the myeloma cells spread into the bone marrow of all the large bones of the body. By the time the disease is diagnosed, most patients have myeloma cells in multiple sites throughout the bone marrow.