Multiple myeloma is a blood cancer that starts in plasma cells, a white blood cell that generally fights infections. Multiple myeloma develops after healthy plasma cells, located in the bone marrow, mutate into cancerous myeloma cells.
The cancerous myeloma cells grow uncontrollably, crowding out new healthy blood cells made in the bone marrow, including red blood cells, white blood cells, and platelets. The cancerous myeloma cells can overtake normal blood cells in the bone marrow, destroy bone tissue, and spread all over the body

.https://www.mountsinai.org/health-library/diseases-conditions/multiple-myeloma