Multiple myeloma. Multiple myeloma cells are abnormal plasma cells (a type of white blood cell) that build up in the bone marrow and form tumors in many bones of the body. Normal plasma cells make antibodies to help the body fight infection and disease. As the number of multiple myeloma cells increases, more antibodies are made. This can cause the blood to thicken and keep the bone marrow from making enough healthy blood cells. Multiple myeloma cells also damage and weaken the bone.