Normally, plasma cells produce different immunoglobulins (antibodies), a part of the body's humoral immune system.

In multiple myeloma, the malignant plasma cell clones divide uncontrollably, producing an abundance of the same immunoglobulin (antibody), called the M-protein or myeloma paraprotein. The rapidly growing number of myeloma cells crowd the bone marrow, destroy bone, and create mass lesions called plamacytomas. The effect of myeloma cells and substances that they produce on bone leads to weak bones and high calcium. The M-protein can clog the kidney and dilutes the function of rest of the humoral immune system leading to bacterial infections.