Multiple myeloma is cancer of the plasma cells in bone marrow. Diseases in which plasma cells become cancerous are known as plasma cell neoplasms. Plasma cells are part of the immune system, a network of specialized cells that make proteins (antibodies) that attack and destroy foreign substances. These substances include viruses, bacteria, fungi and parasites.

The cells of the immune system work together to defend the body from infections and other diseases. The main type of immune system cell is the lymphocyte. The two types of lymphocytes are:

• B lymphocytes (B-cells). B-cells defend the body from invading bacteria and other harmful substances by changing into plasma cells, which produce antibodies. The antibodies then mark the foreign substances for destruction.
  
  
• T lymphocytes (T-cells). T-cells recognize the infected cells and destroy them directly.

When B-cells react to an infection, they mature and transform into plasma cells. Mainly located in the bone marrow (the soft, inner component of bone), plasma cells manufacture and release antibodies, also known as immunoglobulins. When these cells grow uncontrollably, they can produce a tumor, which typically develop in the bone marrow. When only one tumor develops, it is known as a solitary plasmacytoma. More frequently, there are numerous tumors or extensive cellular infiltration spread throughout the bone marrow in many bones of the body. This disorder is known as multiple myeloma.

All forms of blood cells are produced in the bone marrow, including:

• White blood cells (WBCs or leukocytes). Cells that fight infection, which include plasma cells.
  
  
• Red blood cells (RBCs orerythrocytes). Cells that carry oxygen to tissues throughout the body.
  
  
• Platelets (thrombocytes). Cells that help develop blood clots and control bleeding.

In patients with multiple myeloma, the overgrowth of plasma cells can interfere with the bone marrow’s ability to produce adequate numbers of normal cells. Over time, this can result in a shortage of white blood cells (leukopenia), red blood cells (anemia), and platelets (thrombocytopenia). Shortages in these types of cells can lead to a decreased ability to fight off infection, fatigue and excessive bleeding.  Although many plasma cells are being produced in patients with multiple myeloma, the cells are abnormal. As a result, the antibodies they produce are not useful in fighting infections.

The American Cancer Society (ACS) predicts that in 2007 there will be nearly 20,000 new cases of multiple myeloma in the United States. The disease is more common in men than women, with about 10,900 cases occurring in males and 8,900 occurring in females. In 2007, about 10,800 Americans are expected to die of multiple myeloma.

The five-year survival rate for multiple myeloma is estimated at 33 percent by the ACS. Younger patients tend to have a better chance of survival than elderly people. In addition, the five-year survival rate is based on patients who were diagnosed and treated for the disease more than five years ago. Treatment advances may result in a better prognosis for recently diagnosed patients.