Multiple myeloma is a cancer of the plasma cells (derivative cells of the lymphoid system and normally a part of the bone marrow mix of cells - their normal function is to make antibodies against various specific foreign protein antigens). Specifically, each patient’s plasmacytoma/myeloma has arisen from one malignant clone, testified by the fact that the immunoglobulin/antibody secreted from that myeloma is identical from each cell; indeed, this is the best evidence in the subject of Oncology that cancer does have a single cell/clone origin. Where the clone of cells remains localised to one site we refer to the disease as plasamacytoma, whereas, when there are many lesions due to spread of the malignancy to multiple bones and marrow, we refer to the disease as multiple myeloma.
In 56% of cases of myeloma, the immunoglobulin is IgG, it is IgA in 27%, IgD in 1.5% and IgM in 0.2%. In 17% of cases, there is no whole immunoglobulin secreted but a Bence Jones protein (a part of the immunoglobulin molecule, specifically the light chain part) is detectable in the urine. The myeloma is then termed IgG myeloma, IgA myeloma etc.