Which is a malignant disorder of mature antibody-secreting b lymphocytes or plasma cells?

The malignant disorder of mature antibody-secreting b lymphocytes or plasma cells is called the Multiple Myeloma.

Multiple Myeloma is a cancer of plasma cells in the bone marrow. The condition features abnormal proliferation of plasma cells in the bone marrow, destructive bone lesions, and the production of abnormal proteins, specifically antibodies. Its symptoms may include, fatigue, bone pain, bone fracture, kidney failure among other symptoms.