Multiple myeloma is a type of cancer formed by malignant plasma cells. Normal plasma cells are an important part of the immune system.

The immune system is composed of several types of cells that work together to fight infections and other diseases. Lymphocytes (lymph cells) are the main cell type of the immune system. There are 2 types of lymphocytes: T cells and B cells.

When B cells respond to an infection, they mature and change into plasma cells. Plasma cells live mainly in the bone marrow. Bone marrow is the soft, inner part of bones. The role of plasma cells is to produce and release proteins called antibodies (or immunoglobulins) to attack and help kill disease-causing germs such as bacteria and viruses.

When plasma cells grow out of control, they can produce a tumor. These tumors generally develop in the bone marrow. If there is only one tumor, it is called a plasmacytoma. Usually, the plasma cell tumors are spread throughout the bone marrow and then they are referred to as myeloma or multiple myeloma.

The overgrowth of plasma cells can interfere with the normal blood-forming functions of the bone marrow. This can result in a shortage of red blood cells, a condition called anemia. Anemia causes fatigue. A shortage of blood platelets (cells that seal damaged blood vessels) can also occur. This can lead to excessive bleeding after cuts or scrapes. Another problem caused by an excess of plasma cells is leukopenia, a condition in which there is a shortage of normal infection-fighting white blood cells. A shortage of these cells causes decreased resistance to infections.

The myeloma also reacts with bone-dissolving cells. There are 2 major kinds of bone cells. Osteoblasts make bone and osteoclasts dissolve it. These 2 kinds of cells work together to model each bone into its proper shape. Myeloma cells make a substance that stimulates osteoclasts and speeds up the dissolving of bone. Weakened and even fractured bones are a major problem in people with myeloma.

The abnormal plasma cells do not protect the body from infections. As mentioned before, normal plasma cells produce antibodies that attack specific infectious agents. For example, if you developed pneumonia, normal plasma cells would produce antibodies that specifically attack and kill this type of bacteria. However, the antibodies produced by myeloma cells are not helpful in fighting infections.

Sometimes, before multiple myeloma develops, it is preceded by another condition of excessive plasma cell growth. This condition is called monoclonal gammopathy of undetermined significance.

Monoclonal Gammopathy of Undetermined Significance (MGUS)

In monoclonal gammopathy of undetermined significance (MGUS), abnormal plasma cells produce excess amounts of antibody protein. However, these plasma cells do not form an actual tumor or mass and do not cause any symptoms. MGUS usually does not affect a person's health. In particular, it doesn't cause bone weakening. It is found because a routine blood test finds a high level of protein in the blood and further testing shows the protein is immunoglobulin.

With time, many people with MGUS eventually develop multiple myeloma, lymphoma, or a disease called amyloidosis. The rate of this happening is about 1% per year. The risk of this happening is higher in people whose protein levels are particularly high. Patients with MGUS usually need frequent medical examinations and tests to detect possible progression to multiple myeloma, but they do not need immediate treatment.

Recently, scientists have studied the genes of the plasma cells in patients with MGUS. They found that the genetic make-up of these plasma cells resembles myeloma plasma cells more than it resembles normal plasma cells. This suggests that these cells are truly malignant, not just slow growing. Because, in general, people with MGUS are elderly, they may not live long enough for their MGUS to transform into myeloma.

Solitary Plasmacytomas

This is another category of plasma cell growth. Rather than multiple tumors in different locations as in multiple myeloma, there is only 1 tumor, hence the name "solitary" plasmacytomas.

Solitary plasmacytomas develop in bone marrow, or they may start in tissues other than bone marrow (such as the lungs or the lining of the sinuses, throat, or other organs). This is called extramedullary disease, meaning outside the bone marrow. These tumors are treated by radiation therapy and/or sometimes with surgery. The patient's outlook for recovery or survival is usually excellent if no other plasmacytomas are found later on. However, most people with solitary plasmacytoma will develop multiple myeloma, especially if the plasmacytoma were in bone, and therefore these people need frequent examinations and tests to detect this progression as early as possible. Early treatment may lead to an improved outcome.

Revised: 08/04/2006