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. The immune system is mainly made up of lymphocytes. The two main types of lymphocytes are T cells and B cells. When an infection occurs in the body, B cells react by developing into plasma cells.