Multiple myeloma is a form of cancer that is caused by the malignant and abnormal growth of plasma cells.
Plasma cells are immune cells typically found in the bone marrow that specialize in the production of antibodies, proteins that help the body to fight off infection. When plasma cells undergo rapid abnormal growth, they can form tumors called plasmacytomas. Multiple myeloma is characterized by the presence of multiple tumors. The presence of only one plasmacytoma would suggest a different disease known as isolated plasmacytoma.