Binding polypeptides that specifically bind BLyS protein or BLyS-like polypeptides can be used in methods of the invention for detecting, diagnosing, or prognosing a disease or disorder associated with aberrant BLyS or BLyS receptor expression or inappropriate function of BLyS or BLyS receptor, comprising BLyS binding polypeptides or fragments or variants thereof, that specifically bind to BLyS. The present invention further relates to methods and compositions for preventing, treating or ameliorating a disease or disorder associated with aberrant BLyS or BLyS receptor expression or inappropriate BLyS function or BLyS receptor function, comprising administering to an animal, preferably a human, an effective amount of one or more BLyS binding polypeptides or fragments or variants thereof, that specifically bind to BLyS.