Graves’ disease is a disorder of the immune system. It is not known why the lymphocytes—white blood cells involved in the body's protective defenses—begin to attack the body's own tissues. When lymphocytes attack the thyroid gland, it responds by producing too much thyroid hormone.raves’ disease is a disorder of the immune system. It is not known why the lymphocytes—white blood cells involved in the body's protective defenses—begin to attack the body's own tissues. When lymphocytes attack the thyroid gland, it responds by producing too much thyroid hormone.