Multiple myeloma is a rare type of cancer that starts in the plasma cells, which are a type of white blood cell. Healthy plasma cells are found in the bone marrow and help the body fight infections by producing antibodies that identify and destroy foreign substances that can harm the body. Plasma cells are essential for the body’s immune system. Plasma cells can become cancerous. When they do, they continue to grow and a tumor develops called a plasmacytoma. Plasmacytomas usually grow within bone, but can sometimes be found in tissue as well.