Multiple myeloma is a cancer of plasma cells. Normal plasma cells are found in the bone marrow and are an important part of the immune system. The immune system is made up of several types of cells that work together to fight infections and other diseases. Lymphocytes are one of the main types of white blood cells in the immune system and include T cells and B cells. Lymphocytes are in many areas of the body, such as lymph nodes, the bone marrow, the intestines, and the bloodstream.