Plasma cells produce antibodies , proteins that move through the bloodstream to help the body get rid of harmful substances. Each type of plasma cell responds to only one specific substance by making a large amount of one kind of antibody. These antibodies find and act against that one substance. Because the body has many types of plasma cells, it can respond to many substances. Cells become abnormal and are produced in large amounts.
Cancerous cells interfere with the growth and functions of normal cells.
In addition, they can spread from one part of the body to another. When cancer involves plasma cells, the body keeps producing more and more of these cells. Myeloma cells tend to collect in the bone marrow and in the hard, outer part of bones. Sometimes they collect in only one bone and form a single mass, or tumor , called a plasmacytoma . In most cases, however, the myeloma cells collect in many bones, often forming many tumors and causing other problems.