If unwanted antibodies are present in the patients serum, could it be a source of danger to the patient?

Depends on the...: ...Definition of "unwanted." in autoimmune diseases, your immune system makes antibodies (abs) that can attack your own body and cause damage. In multiple myeloma, the malignant immune cells make way to much ab; the danger may not be from the ab but from the cancer itself. Finally, your immune system may make abs to things it saw a long time ago that are no longer relevant, but not dangerous.