A protein that is made by B cells and plasma cells (types of white blood cells) and helps the body fight infection. Some Igs may be found in higher than normal amounts in patients with certain conditions or certain types of cancer, including multiple myeloma and Waldenstrom macroglobulinemia. Measuring the amount of specific Igs in the blood and urine may help diagnose cancer or find out how well treatment is working or if cancer has come back. Some Igs may be used as tumor markers. Also called immunoglobulin.